The "fungal filter?"

You saw the title.

What exactly am I talking about here? 



Today, I want to double back and talk a bit about our gooey friends, the fungi- for just a few minutes. Despite their off-putting appearance to some, they may be among the most beautiful, elegant, and useful organisms we encounter in the aquatic world.

Why do I have such devotion to organisms which most of us find truly revolting in appearance? 

Because they are among the most important and useful organisms which we can have in our botanical method aquariums. Think about how they arrive in aquatic ecosystems, what they consume, how they derive nutrition, and what they do for the overall ecosystem.

As everyone knows, when you put stuff in water, one of four things seems to happen:

1) Nothing.

2) It gets covered in a gooey slime of fungal growth, and "biofilm."

3) It starts to break down and decompose.

4) Both 2 and 3

Now, it's pretty much a "given" that any botanicals or leaves that you drop into your aquarium will, over time, break down. Wood, too. And typically, before they break down, they'll "recruit" (a fancy word for "acquire') a coating of some rather unsightly-looking growth. Well, "unsightly" to those who have not been initiated into our little world of decomposition, fungal growth, biofilms, tinted water, etc., and maintain that an aquarium by definition is a pristine-looking place without a speck of anything deemed "aesthetically unattractive" by the masses! 

So, with that little explanatory passage out of the way, let's take a closer look at fungi-the stuff that you'll see covering the leaves, botanicals, and wood that you place into your aquarium, and why you actually WANT the stuff there in the first place.

The fungi known as aquatic hyphomycetes produce enzymes which break down botanical materials in water. Essentially, they are primary influencers of leaf maceration. They're remarkably efficient at what they do, too. In as little as 3 weeks, as much as 15% of the decomposing leaf biomass in many aquatic habitats is "processed" by fungi, according to one study I found!

Aquatic hyphomycetes play a key role in the decomposition of plant litter of terrestrial origin- an ecological process in rain forest streams that allows for the transfer of energy and nutrients to higher tropic levels. 

This is what ecologists call "nutrient cycling", folks.

These fungi colonize leaf litter and twigs and such soon after they're immersed in water. The fungi mineralize organic carbon and nutrients and convert coarse particulate matter into fine particulate organic matter. They also increase leaf litter palatability to shredders, which helps facilitate physical fragmentation.

Fungi tend to colonize wood and botanical materials, because they offer them a lot of surface area to thrive and live out their life cycle. And cellulose, hemicellulose, and lignin- the major components of wood and botanical materials- are degraded by fungi, which posses enzymes that can digest and assimilate these materials and their associated organics!

Fungi are regarded by biologists to be the dominant organisms associated with decaying leaves in streams, so this gives you some idea as to why we see them in our aquariums, right?

In aquarium work, we see fungal colonization on wood and leaves all the time. Most hobbyists will look on in sheer horror if they saw the same extensive amount of fungal growth on their carefully selected, artistically arranged wood pieces as they would in virtually any aquatic habitat in Nature!

Yet, it's one of the most common, elegant, and beneficial processes that occurs in natural aquatic habitats!

It's everywhere.

Of course, fungal colonization of wood and botanicals is but one stage of a long process, which occurs in Nature and our aquariums. And, as hobbyists, once we see those first signs of this stuff, the majority of us tend to reach for the algae scraper or brush and remove as much of it as possible- immediately! And sure, this might provide some "aesthetic relief" for some period of time- but it comes right back...because these materials will provide a continuous source of food and colonization sites for fungal growths for some time!

I know that the idea of "circumventing" this stuff is appealing to many, but the reality is that you're actually interrupting an essential, ecologically beneficial natural process. And, as we know, Nature abhors a vacuum, and new growths will return to fill the void, thus prolonging the process.

Again, think about the role of aquatic hyphomycetes in Nature.

Fungal colonization facilitates the access to the energy trapped in deciduous leaves and other botanical materials found in tropical streams for a variety of other organisms to utilize. 

As we know by now, fungi play a huge role in the decomposition of leaves, both in the wild and in the aquarium. By utilizing special enzymes, aquatic fungi can degrade most of the molecular components in leaves, such as cellulose,, hemicelluloses, starch, pectin and even lignin.

Fungi, although not the most attractive-looking organisms, are incredibly useful...and they "play well" with a surprisingly large number of aquatic life forms to create substantial food webs, both in the wild and in our aquariums!

Natural habitats are absolutely filled with this stuff...It's like the whole game here- an explosion of life-giving materials, free for the taking...

Yet, we freak the fuck out about it when it shows up.

Another "mental shift", I which many of you have already made, no doubt. I certainly look forward to seeing many examples of us utilizing "what we've got" to the advantage of our fishes! AGAIN: A truly "Natural" aquarium is not sterile. It encourages the accumulation of organic materials and other nutrients- not in excess, of course.

The love of pristine, sterile-looking tanks is one of the biggest obstacles we need to overcome to really advance in the aquarium hobby, IMHO.

Biofilms, fungi, algae...detritus...all have their place in the aquarium. Not as an excuse for lousy or lazy husbandry- but as supplemental food sources to "power" the ecology in our tanks.

And of course, as we've discussed many times here, fungi are actually an important food item for other life forms in the aquatic environments tha we love so much!  In one study I stumbled across, gut content of over 100 different aquatic insects collected from submerged wood and leaves showed that fungi comprised part of the diet of more than 60% of them, and, in turn, aquatic fungi were found in gut content analysis of many species of fishes!

One consideration: Bacteria and fungi that decompose decaying plant material in turn consume dissolved oxygen for respiration during the process.

This is one reason why we have told you for years that adding a huge amount of botanical material at one time to an established, stable aquarium is a recipe for disaster. There is simply not enough fungal growth or bacteria to handle it. They reproduce extremely rapidly, consuming significant oxygen in the process.

Bad news for the impatient.

Support. Co-dependency. Symbiosis. Whatever you want to call it- the presence of fungi in aquatic ecosystems is extremely important to other organisms.

You can call it free biological filtration for your aquarium!

GREAT news for the patient, the studious, and the accepting.

Think about this: These life forms arrive on the scene in Nature, and in our tanks, to colonize appropriate materials, to process organics both in situ on the things that they're residing upon (leaves, twigs, branches, seed pods, wood, etc.). 

Yeah, if you intervene by removing stuf-f bad things can happen. Like, worse things than just a bunch of gooey-looking fungal and biofilm threads on your wood. Your aquarium suddenly loses its capability of processing the leaves and associated organics, and- who's there to take over? 

Okay, I'm repeating myself here- but there is so much unfounded fear and loathing over aquatic fungi that someone has to defend their merits, right? Might as well be me!

My advice; my plea to you regarding fungal growth in your aquarium? Just leave it alone. It will eventually peak, and ultimately diminish over time as the materials/nutrients which it uses for growth become used up. It's not an endless "outbreak" of unsightly (to some) fungal growth all over your botanicals and leaves. It goes away significantly over time. 

"Over time."

That's "Fellman Speak" for "Please be more fucking patient!"

Seriously, though, hobbyists tend to overly freak out about this kind of stuff. Of course, as new materials are added, they will be colonized by fungi, as Nature deems appropriate, to "work" them.

It's one of those things in the botanical-method aquarium that we need to wrap our heads around. We need to understand, lose our fears, and think about the many positives these organisms provide for our tanks. These small, seemingly "annoying" life forms are actually the most beautiful, elegant, beneficial friends that we can have in the aquarium. When they arrive on the scene in our tanks, we should celebrate their appearance.


Because their appearance is yet another example of the wonders of Nature playing out in our aquariums, without us having to do anything of consequence to facilitate their presence, other than setting up a tank embracing the botanical method in the first place. We get to watch the processes of colonization and decomposition occur in the comfort of our own home. The SAME stuff you'll see in any wild aquatic habitat worldwide.


For those of you who MUST find some familiar comfort in established philosophy- look no further than the beloved master, Takashi Amano.  He laid down this track decades ago...

Yup. I'm channeling Mr. Amano here. 

In the botanical method aquairum, Amano's concept of embracing the Japanese philosophy of wabi-sabi takes over. Accepting the transient nature of things and enjoying the beauty of the changes that occur over time. 

Part of the game, as we've discussed ad naseum here, is to understand, appreciate, and ultimately embrace the way the aquatic environment is influenced by the fungal growths, biofilms, and decomposition which occurs when botanicals are added into our aquariums. 

Remember, your aquairum is not a pice of kinetic art. It's a miniature, closed aquatic ecosystem. Processes which occur in Nature play out daily in your tank.

Yeah, I admit, decades ago, I freaked out about seeing fungal growths in my tanks, too. I'd get a bit scared, wondering if something was wrong, and why no one else's aquariums ever seemed to look like mine. I used to think something was really wrong!

To reassure myself, I would stare for hours at underwater photos taken in the Amazon region, showing decaying leaves, biofilms,and fungi all over the leaf litter. I'd read the studies by researchers like Henderson and Walker, detailing the dynamics of wild leaf litter zones and how productive and unique they were.

I remember telling myself that what I was seeing in my tanks was remarkably similar to what I saw in images and videos of wild aquatic habitats that I wanted to replicate. They seem to look- and even function- so similarly.

I'd pour over my water quality tests, confirming for myself that everything was okay. It always was. And of course I would watch my fishes for any signs of distress...

I never saw them.

Truth be known, I knew that there wouldn't be any issues, because I created my aquariums with a solid embrace of basic aquatic biology; an understanding that an aquarium is not some sort of underwater art installation, but rather, a living, breathing microcosm of organisms which work together to create a biome..and that the appearance of the aquarium only tells a small part of the story.


And another big concept for you to wrap your head around:

Your aquarium- or more specificlally- the colonized botanical materials which comprise the botanical-method aquarium "infrastructure" acts as a biological "filter system."

In other words, the botanical materials present in our systems provide enormous surface area upon which beneficial bacterial biofilms and fungal growths can colonize. These life forms, like fungi, utilize the organic compounds present in the water as a nutritional source, thus creating a "nutrient assimilation process."

Understanding and embracing this has changed everything about how I look at aquarium management and the creation of functional closed aquatic ecosystems. 

It's really put the word "natural" back into the aquarium keeping parlance for me. The idea of creating a multi-tiered ecosystem, which provides a lot of the requirements needed to operate successfully with just a few basic maintenance practices, the passage of time, a lot of patience, and careful observation.

It means adopting a different outlook, accepting a different, yet very beautiful aesthetic. It's about listening to Nature instead of the "influencer" on YouTube with the flashy, gadget-driven tank and nothing substantive to back up his vapid narrative. It means educating yourself a bit. It's not always fun at first for some, and it initially seems like you're somehow doing things wrong.

But you're not. And Mother Nature won't let you down if you don't lose faith in Her.

And yeah- it's about faith. Faith in Mother Nature, who's been doing this stuff for eons. She's got this. She'll hook you up...If you allow Her. If you have faith in Her processes.

Have faith.

Stay bold. Stay thoughtful. Stay curious. Stay patient...

And Stay Wet.


Scott Fellman

Tannin Aquatics 


Throwing some light on blackwater...

It goes without saying that the single most important component of our aquariums is also the most obvious...water! As the literal bearer of life and the environment in which our fishes, plants, and other organisms thrive, it's fundamental. it's the reason we're drawn to fishes, not gerbils, Tarantula, or Mice- or whatever other pets people keep!

Yeah, we're into water!

And I dear say that we take it for granted a bit.

Now, sure, some hobbyist rightfully place the importance of good quality, properly-conditioned water at the very top of their "want list" of "Stuff" required for successful aquariums. These are often fish breeders and very serious hobbyists, who understand the fundamental importance of good water for their work.

Some of the most common questions we receive lately are "How much _______ do I need to get my water to look like________?" or "How much_______ is needed to lower the pH in my tank?" Or, "How much do I need to get a good amount of humic substances and tannins into my aquarium?"

I usually respond with a simple, "I don't know."

These are all really good questions. Logical. Important.  I kind of feel like many hobbyists are looking for a plug-and-play "formula" or "recipe" for how to accomplish certain water-conditioning tasks.

I totally get that. But the reality is...there IS no "recipe" for how to do this stuff.

And it sucks, I know.

"Why, Scott? I read that you can just add some of this blackwater extract that you can buy online, and maybe add some catappa leaves, and..."

Stop. STOP. Please, we're just making this painful.

Simply adding leaves or bottled extracts to your tap water isn't going to result in "Instant Amazon" or whatever. There are numerous complexities and nuances which contribute to these habitats that to simply recommend adding "X" to your water isn't the whole story.

There are so many variables in the equation that it's almost impossible to give a definitive answer. And yeah, us guys in the botanical biz haven't really helped the situation. Over the years, vendors who sold catappa leaves, for example, would recommend starting amounts ("three leaves per 15 liters of water" or whatever...) of botanical materials to use in aquariums.

I mean, we've sort of done it, too...And, although our recommended "dosage" of leaves was given for different reasons (to avoid adding too much material to your tank too quickly), the idea of a "recipe" in general is kind of delusional, IMHO.

Now, this was all well and good, but it's based on....what? I mean, is this based on how many leaves of _______ size that a typical hobbyist with a 10-gallon aquarium needs to get the water "looking brown?" Or to lower tap water with a starting pH of 7.4 and a KH of ___ to pH of 6.9? Or to impart "x" ppm of tannins or humic substances into this given quantity of water?

See? Add to this story the fact that you really can't soften water and make it more "malleable" by using botanicals or extracts alone, and you've got a good case for confusion! It's just not that simple.

Maybe we can gain a bit of understanding- or at least, an appreciation for the dynamics of this process, by looking once again to Nature.

Have you very thought about how water reaches all of the wild aquatic systems of the world? I mean, it's got to get there some way, right? So, how does it reach the ponds, lakes, streams, and rivers and forest floors of the world?

Well, some simply falls into the body of water directly from the sky, and that's that. Some is a result of other overflowing streams and rivers (like, ya' know- those flooded Igapo forests we talk about!). Inputs of precipitation falling over the area of an aquatic habitat are transferred to the habitat via a number of different pathways.

It's surprisingly complicated.

There's like a whole field of science devoted to studying this process! It's called Hydrology, and it's incredibly interesting...As fish geeks, we're probably already acquainted with this field of study, at least tangentially! 

So, water comes from a variety of sources, reaching a myriad of ecological niches. However, not all of the water has such an easy journey on its way into our favorite aquatic habitat!

Even in the case of rainwater, some of it simply lands on tree leaves in the surrounding area and evaporates. This is a process scientists call "interception", and accounts for the fact that not all water makes it to the ground. Water that does reach the ground enters the soil through a process called infiltration. slowly percolating down to soil areas known as the "saturated zone"- and as you'd imagine, this is where the fun really begins! (to a soil geologist, at least!)

The soil properties control the infiltration capacity; these include things like soil permeability, the presence of vegetation and plant roots, and how much water is already in the soil. Through what is known as "ground water flow", ultimately, the water finds it way into our favorite aquatic habitats. It's important to note that soil texture ( the relative proportion of sand, silt and clay particles within the mix) affects infiltration rates. 

Sandy soils like the "podzols", common to forested areas of South America that we've talked about have higher permeability than some clay-based soils. In some really arid areas a "crust" can form on the soil surface, decreasing the permeability. And of course, the thickness of the soil directly affects how much water the soil can actually absorb.

And, in many cases, the substrate composition and its relationship with water has direct impact on the life forms which inhabit these aquatic systems. In the case of some habitats, like vernal pools, which are filled with water seasonally, the substrate is of critical importance to the aquatic life forms which reside there.

Yeah, soils and geology are perhaps the primary driver of water composition in Nature. 

Let's talk more about "blackwater."

In a blackwater environment, the color is a visual indicator of an influx of dissolved materials that contribute to the "richness" of the environment. Indeed, a blackwater environment is typically described as an aquatic system in which vegetation decays, creating  tannins that leach into the water, making a transparent, acidic water that is darkly stained, resembling tea.

But, that's not the whole story, really.

It’s important to really try to understand the most simple of questions- like, what exactly is “blackwater”, anyways?

A scientist or ecologist will tell you that blackwater is created by draining from older rocks and soils (in Amazonia, look up the “Guyana Shield”), which result in dissolved fulvic and humic substances, present small amounts of suspended sediment, and characterized by lower pH (4.0 to 6.0) and dissolved elements, yet higher SiOcontents. Magnesium, Sodium, Potassium, and Calcium concentrations are typically very low in blackwater. Electrical conductivity (ORP) is also lower than in so-called "whitewater" habitats.

Tannins are also imparted into the water by leaves and other botanical materials which accumulate in these habitats.


The action of water upon fallen leaves and other botanical-derived materials leaches various compounds out of them, creating the deep tint that many of us are so familiar with. Indeed, this "leaching" process is analogous to boiling leaves for tea. The leached compounds are both organic and inorganic, and include things like tannin, carbohydrates, organic acids, pectic compounds, minerals, growth hormones, alkaloids, and phenolic compounds.

Most of the of the extractable substances in the surface litter layer are humic acids, typically coming from decaying plant material. Scientists have concluded that greater input of plant litter leads to greater input of humic substances into ground water.

In other words, those leaves that accumulate on the substrate are putting out significant amounts of humic acids, as we've talked about previously! And although humic substances, like fulvic acid, are found in both blackwater and clear water habitats, the organic detritus (you know, from leaves and such) in blackwater contains more extractable fulvic acid than in clearwater  habitats, as one might suspect!

The Rio Negro, for example, contains mostly humic acids, indicating that suspended sediment selectively adsorbs humic acids from black water.  The low concentration of suspended sediments in rivers like the Rio Negro is one of the main reasons why high concentrations of humic acids are maintained. With little to no suspended sediment, there is no "adsorbent surface" (other than the substrate of the river, upon which these acids can be taken hold of (adsorb).

When you think about it, all of this this kind of contributes to why blackwater has the color that it does, too. Blackwater in the Amazon basin is colored reddish-brown. Why? Well, it has  those organic compounds dissolved in it, of course. And most light absorbtion is in the blue region of the spectrum, and the water is almost transparent to red light, which explains the red coloration of the water!

And many of those organic compounds come from the surrounding land, as touched on above...

In summary, natural "blackwaters" typically arise from highly leached (tropical) environments where most of the soluble elements in the surrounding rocks and soils are rapidly removed by heavy rainfall. Materials such as soils are the primary influence on the composition of blackwater.

Leaves and other materials contribute to the process and appearance in Nature, but are NOT the primary “drivers” of its creation and composition.


So, right from the start, it’s evident that natural blackwater is “all about the soils…” Yeah, I'll repeat it again: It’s more a product of geology than just about anything else. 

More confusing, recent studies have found that most of the acidity in black waters can be attributed to dissolved organic substances, and not to dissolved carbonic acid. In other words, organic acids from compounds found in soil and decomposing plant material, as opposed to inorganic sources. Blackwaters are almost always characterized by high percentages of organic acids.

Despite the appearance, as a general rule, blackwater rivers are lower in nutrients than clear rivers. Wouldn't it be interesting, when contemplating more natural biotope/biotype aquariums, to study and take into consideration the surrounding geology and physical characteristics of the habitat?  Too recreate the habitat based on the soil or geological composition of the surrounding terrestrial environment?

As we know now, the influence of factors like soil, and the presence of terrestrial materials like seed pods, leaves, and branches play a huge role in the chemical composition and appearance-of the water. It's really no different in the aquarium, right?

Like so many things in nature, the complexity of blackwater habitats is more than what meets the eye. Chemically, biologically, and ecologically, blackwater habitats are a weave of interdependencies- with soil, water, and surrounding forest all functioning together to influence the lives of the fishes which reside within them. No single factor could provide all of the necessary components for fish populations to thrive.

To damage or destroy any one of them could spell disaster for the fishes- and the ecosystem which supports them. It is therefore incumbent upon us to understand, protect, and cherish these precious habitats, for the benefit of future generations. 

And with regards to our aquarium work?

Although there may even be breakthroughs in terms of blackwater extracts and additives coming to market, there are still a lot of questions that would have to be answered before we could simply state that "X" drops per gallon of such an such a formula would yield a specific outcome. This reminds me of the reef aquarium world more an more, lol.

So, if I've made any "argument" here, it's that this stuff is every bit as much of an "art"- in terms of aquarium keeping- as it is a "science." We will, at least for the foreseeable future, have to use the data we have available and formulate a best guess as to how much of what can give us some of the impacts we are interested in for our aquariums.

We simply can't authoritatively make blanket statements like, "You need to use "X" catappa leaves per gallon in order to recreate Rio Negro-like conditions in your aquarium!" We can't simply state that you can throw in some podzolic soil and achieve blackwater, either. There are many factors in play, as we've discussed here, right?

Marketing hyperbole aside, we really are sort of...guessing.

And that's certainly nothing to be discouraged about!

We, as a community, are getting deeper into the functional aspects of blackwater, botanical-style aquariums than ever before. More light is being shed on what's going on in both our aquariums and in the natural habitats we desire to replicate. We are learning more every day about how the presence of tannins and humic substances in our aquariums is affecting the health, longevity, and spawning behaviors of our blackwater fishes.

We're learning about the challenges and realities of managing blackwater systems over the long term- understanding the good, the bad, and the dangerous possibilities that are present when we experiment with these ideas.

There is much, much more work to be done..And a lot of talented hobbyists like yourself are out there on the front lines every day, contributing to the body of knowledge that will benefit the hobby for generations!

Stay persistent. Stay bold. Stay open-minded. Stay curious. Stay disciplined...

And Stay Wet.


Scott Fellman

Tannin Aquatics 


Botanical materials in the wild...and in the aquarium: Influences and Impacts.

As you know, we've spent the better part of the past 6 years talking about every aspect of the botanical-style aquarium that we can think of. We've talked about techniques, approaches, ideas, etc. And we've spent a lot of time sharing information about wild aquatic habitats that we might be interested in replicating in both form and function.

However, I think we haven't spent as much time as we should talking about how botanicals "behave" in wild aquatic habitats.  Much of this stuff has implications for those of us who are interested in replicating these habitats in our aquariums. So, let's dive in a bit more on this topic today!

Among the trees of the flooded forests, after the fruits mature (which occurs at high water levels), seeds will fall into the water and may float on the surface or be submerged for a number of weeks. Ecologists believe that the seed production of the trees coincides with the flood pulses, which facilitates their dispersal by water movement, and by the actions of fish.

Interestingly, scientists postulate that these floating or sinking seeds, which  germinate and establish seedlings after the flood waters recede, do very well, sprouting and establishing themselves quickly, and are not severely affected by waterlogging in most species.

So, within their cycle of life, the trees take advantage of the water as part of their ecological adaptation. Trees in these areas have developed specialized morphologies, such as advantageous roots, butress systems and stilt roots.

 In a lot of wild aquatic habitats where leaf litter and other allochthonous materials accumulate, there are a number of factors which control the density, size, and type of materials which are deposited in streams and such. The flow rate of the water within these habitats determines a lot of things, such as the size of the leaves and other botanical materials  and where in the stream they are deposited. 

I often wonder how much the fallen leaves and seed pods impact the water chemistry in a given stream, pond, or section of an Amazonian flooded forest. I know that studies have been done in which ecologists have measured dissolved oxygen and conductivity, as well as pH. However, those readings only give us so much information.

We hear a lot of discussion about blackwater habitats among hobbyists, and the implications for our aquariums. And part of the game here is understanding what it is that makes this a blackwater river system to begin with. We often hear that blackwater is "low in nutrients." 

What exactly does this mean?

One study concluded that the Rio Negro is a blackwater river in large part because the very low nutrient concentrations of the soils that drain into it have arisen as a result of "several cycles of weathering, erosion, and sedimentation." In other words, there's not a whole lot of minerals and nutrients left in the soils to dissolve into the water to any meaningful extent!

Black-waters, drain from older rocks in areas like the Negro river, result from dissolved fulvic and humic substances, present small amounts of suspended sediment, lower pH (4.0 to 6.0) and dissolved elements. Yes, highly leached tropical environments where most of the soluble elements are quickly removed by heavy rainfall.

Perhaps...another reason (besides the previously cited limitation of light penetration) why aquatic plants are rather scare in these waters? It would appear that the bulk of the nutrients found in these blackwaters are likely dissolved into the aquatic environment by decomposing botanical materials, such as leaves, branches, etc.

Why does that sound familiar?

Besides the color, of course, the defining characteristics of blackwater rivers are pH values in the range of 4-5, low electrical conductivity, and minimal mineral content. Dissolved minerals, such as  Ca, Mg, K, and Na are negligible. And with these low amounts of dissolved minerals come unique challenges for the animals who reside in these systems.

How do fishes survive and thrive in these rather extreme habitats?

It's long been known that fishes are well adapted to their natural habitats, particularly the more extreme ones. And this was borne out in a recent study of the Cardinal Tetra. Lab results suggest that humic substances  protect cardinal tetras in the soft, acidic water in which they resides by preventing excessive sodium loss and stimulating calcium uptake to ensure proper homeostasis.

This is pretty extraordinary, as the humic substances found in the water actually enable the fishes to survive in this highly acidic water which is devoid of much mineral content typically needed for fishes to survive!

And of course, botanicals, leaves, and wood typically have an abundance of these humic substances, right? They are useful for more than just an interesting and unique aesthetic effect! There is a lot of room for research about influencing the overall environment in our aquariums here! I think we've barely scratched the surface of the potential for utilizing botanicals in our aquariums.

This is another one of those foundational aspects of the natural style of aquarium that we espouse. The understanding that processes like decomposition and physical transformation of the materials that we utilize our tanks are normal, expected, and beautiful things requires us to make mental shifts.

Botanical materials don't have nearly as much impact on the water parameters (other than say, conductivity and dissolved oxygen) as the soils do. These waters have high concentrations of humic and fulvic acids derived from sandy "hydromorphic podsols" prevalent in the region.  However, these allochthonous materials have huge impact on the ecology of these systems!

Leaf litter, as one might suspect, is of huge importance in these ecosystems. Especially in smaller tributaries. In one study which I came across, it was concluded that, "The smaller the stream, the more dependent the biota is on leaf litter habitats and allocthonous energy derived directly or indirectly from the forest." (Kemenes and Forsberg)

From the same study, it was concluded that the substrate of the aquatic habitat had significant influence on the feeding habits of the fishes which resided in them: 

"The biomass of allocthonous insectivore increased in channels with a higher percentage of sandy bottom substrate. Detritivorous insectivore biomass, in contrast, increased significantly in channels with a higher percentage of leaf substrate. General insectivores tended to increase in streams with higher proportions of leafy substrate, too.

Whats the implication for us as hobbyists? Well, for one thing, we can set up the benthic environment in our tanks to represent the appropriate environment for the fishes which we want to keep in them. Simple as that!

t's as much about function as anything else. And, about pushing into some new directions. The unorthodox aesthetics of these unusual aquariums we play with just happen to be an interesting "by-product" of theirfunction.

I personally think that almost every botanical-style aquarium can benefit from the presence of leaves. As we've discussed numerous times, leaves are the "operating system" of many natural habitats (ecology-wise), and perform a similar role in the aquarium.

The presence of botanical materials such as leaves in these aquatic habitats is fundamental. Leaves and other botanicals are extremely pervasive in almost every type of aquatic habitat.

In the tropical species of trees, the leaf drop is important to the surrounding environment. The nutrients are typically bound up in the leaves, so a regular release of leaves by the trees helps replenish the minerals and nutrients which are typically depleted from eons of leaching into the surrounding forests.

Now, interestingly enough, most tropical forest trees are classified as "evergreens", and don't have a specific seasonal leaf drop like the "deciduous" trees than many of us are more familiar with do...Rather, they replace their leaves gradually throughout the year as the leaves age and subsequently fall off the trees.

The implication here?

There is a more-or-less continuous "supply" of leaves falling off into the jungles and waterways in these habitats, which is why you'll see leaves at varying stages of decomposition in tropical streams. It's also why leaf litter banks may be almost "permanent" structures within some of these bodies of water!

Our botanical-style aquariums are not "set-and-forget" systems, and require basic maintenance (water exchanges, regular water testing, filter media replacement/cleaning), like any other aquarium.  They do have one unique "requirement" as part of their ongoing maintenance which other types of aquariums seem to nothave: The "topping off" of botanicals as they break down.

The "topping off" of botanicals in your tank accomplishes a number of things: first, it creates a certain degree of environmental continuity- keeping things consistent from a "botanical capacity" standpoint. Over time, you have the opportunity to establish a "baseline" of water parameters, knowing how many of what to add to keep things more-or-less consistent, which could make the regular "topping off" of botanicals a bit more of a "science" in addition to an "art."

In addition, it keeps a consistent aesthetic "vibe" in your aquarium. Consistent, in that you can keep the sort of "look" you have, while making subtle- or even less-than-subtle "enhancements" as desired. 

Yeah, dynamic.

And, of course, "topping off" botanicals helps keeps you more intimately "in touch" with your aquarium, much in the same way a planted tank enthusiast would by trimming plants, or a reefer while making frags. When you're actively involved in the "operation" of your aquarium, you simply notice more. You can also learn more; appreciate the subtle, yet obvious changes which arise on an almost daily basis in our botanical-style aquariums.

I dare say that one of the things I enjoy doing most with my blackwater, botanical-style aquariums (besides just observing them, of course) is to "top off" the botanical supply from time to time. I feel that it not only gives me a sense of "actively participating" in the aquarium- it provides a sense that you're doing something nature has done for eons; something very "primal" and essential. Even the prep process is engaging.

Think about the materials which accumulate in natural aquatic habitats, and how they actually end up in them, and it makes you think about this in a very different context. A more "holistic" context that can make your experience that much more rewarding. Botanicals should be viewed as "consumables" in our hobby- much like activated carbon, filter pads, etc.- they simply don't last indefinitely.

Many seed pods and similar botanicals contain a substance known as lignin. Lignin is defined as a group of organic polymers which are essentially the structural materials which support the tissues of vascular plants. They are common in bark, wood, and yeah- seed pods, providing protection from rotting and structural rigidity.

In other words, they make seed pods kinda tough.

Yet, not permanent.

That being said, they are typically broken down by fungi and bacteria in aquatic environments. Inputs of terrestrial materials like leaf litter and seed pods into aquatic habitats can leach dissolved organic carbon (DOC), rich in lignin and cellulose. Factors like light intensity, mineral hardness, and the composition of the aforementioned bacterial /fungal community all affect the degree to which this material is broken down into its constituent parts in this environment.

Hmm...something we've kind of known for a while, right?

So, lignin is a major component of the "stuff" that's leached into our aquatic environments, along with that other big "player"- tannin.

Tannins, according to chemists, are a group of "astringent biomolecules" that bind to and precipitate proteins and other organic compounds. They're in almost every plant around, and are thought to play a role in protecting the plants from predation and potentially aid in their growth. As you might imagine, they are super-abundant in...leaves. In fact, it's thought that tannins comprise as much as 50% of the dry weight of leaves!


And of course, tannins in leaves, wood, soils, and plant materials tend to be highly water soluble, creating our beloved blackwater as they decompose. As the tannins leach into the water, they create that transparent, yet darkly-stained water we love so much!  

In simplified terms, blackwater tends to occur when the rate of "carbon fixation" (photosynthesis) and its partial decay to soluble organic acids exceeds its rate of complete decay to carbon dioxide (oxidation).

Chew on that for a bit...Try to really wrap your head around it...

And sometimes, the research you do on these topics can unlock some interesting tangential information which can be applied to our work in aquairums...

Interesting tidbit of information from science: For those of you weirdos who like using wood, leaves and such in your aquariums, but hate the brown water (yeah, there are a few of you)- you can add baking soda to the water that you soak your wood and such in to accelerate the leaching process, as more alkaline solutions tend to draw out tannic acid from wood than pH neutral or acidic water does. Or you can simply keep using your 8.4 pH tap water! 

"ARMCHAIR SPECULATION": This might be a good answer to why some people can't get the super dark tint they want for the long term...If you have more alkaline water, those tannins are more quickly pulled out. So you might get an initial burst, but the color won't last all that long...

I think just having a bit more than a superficial understanding of the way botanicals and other materials interact with the aquatic environment, and how we can embrace and replicate these systems in our own aquariums is really important to the hobby. The real message here is to not be afraid of learning about seemingly complex chemical and biological nuances of blackwater systems, and to apply some of this knowledge to our aquatic practice.

It can seem a bit  intimidating at first, perhaps even a bit contrarian to "conventional aquarium practice", but if you force yourself beyond just the basic hobby-oriented material out there on these topics (hint once again: There aren't many!), there is literally a whole world of stuff you can learn about!

It starts by simply looking at Nature as an overall inspiration...

Wondering why the aquatic habitats we're looking at appear the way they do, and what processes create them. And rather than editing out the "undesirable" (by mainstream aquarium hobby standards) elements, we embrace as many of the elements as possible, try to figure out what benefits they bring, and how we can recreate them functionally in our closed aquarium systems.

There are no "flaws" in Nature's work, because Nature doesn't seek to satisfy observers. It seeks to evolve and change and grow. It looks the way it does because it's the sum total of the processes which occur to foster life and evolution.

We as hobbyists need to evolve and change and grow, ourselves.

We need to let go of our long-held beliefs about what truly is considered "beautiful." We need to study and understand the elegant way Nature does things- and just why natural aquatic habitats look the way they do.  To look at things in context.  To understand what kinds of outside influences, pressures, and threats these habitats face.

And, when we attempt replicate these functions in our aquariums, we're helping to grow this unique segment of the aquarium hobby.

Please make that effort to continue to educate yourself and get really smart about this stuff...And share what you learn on your journey- all of it- the good and the occasional bad. It helps grow the hobby, foster a viable movement, and helps your fellow hobbyists!

Stay studious. Stay thoughtful. Stay inquisitive. Stay creative. Stay engaged...

And Stay Wet.


Scott Fellman

Tannin Aquatics