Leaves- the starting point for our obsession: A rather long "primer"...

Oh, of course. It's a new year, and it's been, I dunno- like 2 weeks since I've talked about leaves in our aquariums...so why not discuss this topic one more time! Really, it's a pretty fundamental one for us in the botanical-style aquarium game, because leaves are pretty much the "bread and butter" materials that we work with, right?

We're going to touch on it in a big way, covering the majority of the topics related to using them in aquariums.

Since you're here, you've no doubt seen the pics of all the cool tanks. You've heard the buzz all over social media. This whole "botanicals" thing is apparently o na lot of people's radar in 2020. Super cool! Seems like more and more people are talking about blackwater aquariums, botanicals, and real "natural-style" aquariums...

And YOU want in on the action now.


Hey, who could blame you? This stuff is kind of cool!

Leaves in an aquarium. Seems to make sense, right? And you've heard so much about how beneficial use of leaves might be for overall fish health..

The Catappa leaf is sort of the "vector" for this stuff, so it makes sense to talk about this species first.

The leaves of the Indian Almond tree,  Terminalia catappa, contain a host of interesting chemicals that may provide direct health benefits for tropical fishes. The leaves themselves contain several flavonoids, like kaempferol and quercetin, a number of tannins, like punicalin and punicalagin, as well as a suite of saponins and phytosterols. Extracts of T. catappa have shown some effectiveness against some bacteria, specifically, Plasmodium, and some parasites as well. 


As we all know by now, when Indian Almond leaves are immersed in water, the tannins and humic substances are released, which can lower the pH of the water if their is minimal general hardness. The tannins are what visually tint the water to that beautiful brownish color which we devotedly call “blackwater.”

It has even been theorized by some that the tannins in Catappa leaves are able to reduce the toxicity of heavy metals in aquarium water, essentially binding them up or chelating them- if true, a most interesting benefit for the urban fish keeper, I might add! I think that’s a pretty big supposition, but I suppose it’s possible that it can be true, right?

At this point, you’re probably thinking, ”Okay, Scott. All of that stuff sounds very scholarly, but what exactly are those things and what can they do for my fishes?”


 Just what DOES make these leaves tick?

Well, lets start with the flavonoids. 

Flavonoids have been shown by science to have direct and synergistic antibacterial activity (with antibiotics) and the ability to suppress bacterial virulence factors in a number of research studies. They may also act as chemical "messengers", physiological regulators, and "cell cycle inhibitors", which bodes well for their use as a prophylactic of sorts. Kaempferol, a noted flavonoid, is thought to have anti-inflammatory and antioxidant properties. Could this be why Betta fanciers used them for so many years after fighting their fishes?

Catappa leaves also contain substances known as Saponins, which can be used to enhance penetration of macromolecules, like proteins, into cell membranes. Some are used in vaccines to help stimulate immune responses, so you can imagine some potential benefits here as well, right?

Yeah. possibly?

Phytosterols are interesting for their alleged capacity to reduce cholesterol in humans, but the benefits are probably non-existent for fishes, especially as it pertains to Catappa leaves in the aquarium! I mention them in this piece merely because fishy authors touting the benefits of Catappa leaves love to throw them out there for reasons I cannot grasp! Maybe it just sounds good. Don't know...

Punicalagins act as antioxidants and are the major component responsible for the antioxidant health benefits of fruits, such as pomegranates (You know, the "wonderful," yet really messy fruit that I always hated as a kid...). They are water soluble and have high bioavailability, so it is quite possible that they are of some benefit to fishes!

I found a cool study conducted by fisheries researchers in Thailand on Tilapia, which concluded that Catappa extract was useful at eradicating the nasty exoparasite, Trichodina, and the growth of a couple of strains of Aeromonas hydrophila was also inhibited by dosing Catappa leaf extract! In addition, this solution was shown to reduce the fungal infection in Tilapia eggs. 

For reference, here is the study:


(Chitmanat, C., Tongdonmuan, K., Khanom, P., Pachontis, P. and Nunsong, W. (2005). Antiparasitic, Antibacterial, and Antifungal activities Derived from a Terminalia catappa solution against some Tilapia (Oreochromis niloticus) Pathogens. Acta Hortic. 678, 179-182 DOI: 10.17660/ActaHortic.2005.678.25)

Lots of tough-to-understand stuff there, right?

So, sure, like so many other aquarium-related matters, there are some possible "side stories" to use of Catappa leaves in our tanks. And benefits that are known and perhaps not yet studied. We do know that there are some well-studied positives that can be achieved by using them in the aquarium.

Much is still anecdotal for aquarium use- a "forced fit" based on the known benefits of these compounds for human health...Fishes, of course, are not humans, and much remains to be discovered about their benefits for aquatic use. Personally, I am in favor of thinking about leaves in the context of creating positive overall ecological influences on our aquariums.

With leaves being so pervasive in Nature, there are many advantages to playing with leaves in our aquariums, too, as you've correctly surmised. Leaves accumulate in aquatic habitats, forming physical aggregations as well as influencing the aquatic habitats in which they fall.

In Nature, leaf litter zones comprise one of the richest and most diverse biotopes in the tropical aquatic ecosystem, yet until recent years, they were seldom replicated in the aquarium. I think this has been due, in large part- to the lack of continuous availability of products for the hobbyist to work with, and a real understanding about what this biotope is all about- not to mention, the understanding of the practicality of creating one in the aquarium.

The "usefulness" of this biotope for fishes is best accurately summarized in this interesting except from an academic paper on Amazonian Blackwater leaf-litter communities by biologist Peter Alan Henderson, that is useful for those of us attempting to replicate these communities in our aquaria:

"..life within the litter is not a crowded, chaotic scramble for space and food. Each species occupies a sub-region defined by physical variables such as flow and oxygen content, water depth, litter depth and particle size…

...this subtle subdivision of space is the key to understanding the maintenance of diversity. While subdivision of time is also evident with, for example, gymnotids hunting by night and cichlids hunting by day, this is only possible when each species has its space within which to hide.”

In other words, different species inhabit different sections of the leaf litter, and we should consider this when creating and stocking our biotope systems...

Neat stuff!

So, beyond just creating an aggregation of material which imparts tannins and humic substances into the water, we're creating a little habitat, every bit as interesting, diverse, and complex as any other we attempt to replicate. In the aquarium, you need to consider both practicality AND aesthetics when replicating this biotope. 

What makes leaves fall off the trees in the first place?

Well, it's simple- er, rather complex...but I suppose it's simple, too. Essentially, the tree "commands" leaves to fall off the tree, by creating specialized cells which appear where the leaf stem of the leaves meet the branches. Known as "abscission" cells. for word junkies, they actually have the same Latin root as the word "scissors",  which, of course, implies that these cells are designed to make a cut! 

And, 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. And the rapid nutrient depletion, by the way, is why it's not healthy to burn tropical forests- the release of nutrients as a result of fire is so rapid, that the habitat cannot process it, and in essence, the nutrients are lost forever.

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!

Okay, I ve blabbered enough on this, right? You're hooked on the idea of utilizing leaves in your tank in a big way. Let's get down to "brass tacks"...How do you start? How do you choose which leaves to play with?

That's a question that is kind of difficult for me to even try to answer...What I'd usually tell you when asked is, "It depends." (extremely helpful, I know...) I mean, it's really about what kind of leaves you like, the size of your tank, what kind of effect you're looking for, etc.

If your goal is to add some leaf litter to supplement your hardscape or even to become the primary focus of your aquascape, then you have a wide variety of leaves from which to choose, some of which we offer here at Tannin (shocker, I know!).

We've written extensively about how you can collect and experiment with naturally fallen leaves from other trees- and you should- so I won't get into that here. Rather, let's stick to the leaves you see on our site, on other vendor's sites (you know, the guys who just sell stuff and don't give you all of this good info...😆), and in many aquariums worldwide.


Let's talk about what I call the "Big 6" leaves we play with in the botanical aquarium world: Catappa, Guava, Jackfruit, Texas Live Oak, Mangrove, and Magnolia. Again, there are many other leaves that hobbyists play with from time to time; however, we've found that these particular leaves tend to be the most useful and versatile leaves, each with their own characteristics, appearance, and benefits.

What are my faves?

Hmmm... It does change from time to time; I'll admit that.

Although Catappa is the sentimental hobby favorite, because of its versatility and worldwide "obtainability", my personal favorite at the moment has to be Guava, followed by Yellow Mangrove. Why? Well, Guava just look exotic to me! They're "aesthetically neutral", and, once submerged and "patina"-covered, serve as a "stand-in" for almost any tropical leaf you might find submerged in some of the habitats where fishes are found.

They have a beautiful shape and structure, and they last a reasonably long time submerged- often a month or so before starting to break down and decompose. They impart a lighter, almost golden "tint" to the water, yet offer that exotic aesthetic which makes up for their lesser amount of tannin-imparting capability. (assuming you see that as a "downside", of course!)

Mangrove, on the other hand, will put out a pretty good "tint" into the water, remain more-or-less structurally intact for long periods of time- often 2 months or more- and have beautiful shape and color. They seem to recruit less biofilms than many other leaves. This is a sort of "anecdotal" observation on my part, but I think that it's pretty accurate.

Oh, and in my experience over years of working with mangrove leaves in blackwater/botanical-style aquariums, I can state confidently that there is simply no detectible salt released by these leaves when submerged, despite concerns about these. Hailing as they do from brackish-water habitats, it's something our customers ask a lot.

I've repeatedly checked this with a very accurate digital refractometer and never detected any salt leaching into my freshwater tanks while using these leaves. I think this is partly because the leaves- when attached to the Mangrove tree, serve as a sort of "escape valve" for salt to work its way out of the tree's vascular system. And the preparation process (ie; boiling and soaking)- releases any minute remaining traces of salt in the leaves' tissues.

When I'm asked which of these leaves last the longest when submerged, I must start by telling you that, although many leaves have different durability and tannin-imparting capabilities, your specific water chemistry, fish population, etc. are the major determining factors in how long a given leaf will last submerged in your aquarium. 

In order of "longevity", from least durable to most durable, I'd say it goes something like this:






*Texas Live Oak

Now, it bears mentioning that we offer Texas Live Oak leaves as "leaf litter", a "form factor" which includes some "extra" stuff, like the occasional twig, strand of moss, terrestrial weed, bits of bark, and acorns. This extra material is, in my opinion, a real "bonus", as it provides additional biological "fuel" to power microbial growth, biofilms, fungi, and other life forms. It's perfect for foraging fishes and shrimp to "work."

Oh, and it looks pretty cool, too!

The other major question we receive about leaves is if you should leave them in the tank until they fully break down.

Leaving leaves in situ to fully decay likely reaches a point when the resulting matter and detritus is essentially inert, consisting of the skeletonize sections of leaf tissue which can decay no further. Dead leaves contain largely inert forms of polysaccharides, and are reach in structures like lignin and cellulose. Most of these compounds have little effect on water quality in a well-maintained aquarium, in my experience.

If you're stressed out about the idea of decomposing leaves in your tanks, doing regular water changes can’t hurt, either- right? 

As you know, we've done a lot of experimentation with "leaf-only" aquariums in recent years, testing their long-term viability, and the results have been incredibly encouraging. When properly maintained, they're as stable and easy to manage as any other aquarium.

Leaf litter beds in aquariums are extremely important to the fishes which inhabit them, providing protection, food, and even physical territory, just like they do in Nature.

As you know by now, I personally favor allowing leaves to break down in the aquarium, because they function much as they would in the wild habitats, fostering microbial growth, food sources, and possibly even providing some form of denitrification.


The preparation of leaves is one of the few "controversies" in the botanical aquarium world.

Why, Scott? Why do we boil this stuff?

Well, to begin with, consider that boiling water is used as a method of making water potable by killing microbes that may be present. Most nasty microbes "check out" at temperatures greater than 60 °C (140 °F). For a high percentage of microbes, if water is maintained at 70 °C (158 °F) for ten minutes, many organisms are killed, but some are more resistant to heat and require one minute at the boiling point of water. (FYI the boiling point of water is 100 °C, or 212 °F)...But for the most part, most of the nasty bacteria that we don't want in either our tanks or our stomachs are eliminated by this simple process.

Ten minutes of boiling is "golden", IMHO. Of course, we boil for other reasons, as we'll touch on in a bit.

For one reason, we boil botanicals to kill any possible microorganisms which might be present on them. Leaves have been exposed to rain and dust and all sorts of things in the natural environment which, in the confines of an aquarium, could  introduce unwanted organisms and contribute to the degradation of the water quality.

The surfaces and textures of many leaves lend themselves to retaining dirt, soot, dust, and other atmospheric pollutants that, although likely harmless in the grand scheme of things, are not stuff you want to start our with in your tank.

So, we give all of our botanicals a good rinse.

Then we boil.

Boiling also serves to soften botanicals.

If you remember your high school Botany, leaves, for example, are surprisingly complex structures, with multiple layers designed to reject pollutants, facilitate gas exchange, drive photosynthesis, and store sugars for the benefit of the plant on which they're found. As such, it's important to get them to release some of the materials which might be bund up in the epidermis (outer layers) of the leaf.  As we get deeper into the structure of a leaf, we find the mesophyll, a layer of tissue in which much of photosynthesis takes place.



We use only dried leaves in our botanical style aquariums, because these leaves from deciduous trees, which naturally fall off the trees in seasons of inclement weather, have lost most of their chlorophyll and sugars contained within the leaf structures. This is important, because having these compounds present, as in living leaves, contributes excessively to the bioload of the aquarium when submerged...

And I like to steep the leaves for a bit, too.


I don't think so, but that's just me.

The steep will help break down the tissues a bit to facilitate sinking, eliminate any surface contaminants, and help release some of the remaining sugars and initial tannins bound up in the leaf tissue. Of course, everyone asks if you're eliminating all of the beneficial tannins when you do this.

My answer: No. You re not. They will keep leaching out tannins for quite some time, even after this comprehensive prep process.

Everyone has a different opinion on this; that's just mine. Lately, I admit I've forgone the boiling water in favor of a room-temperature overnight soak, or sometimes, just a heavy rinse in tap water, and then added the leaves to my aquariums. I've encountered no problems, other than a slightly higher "buoyancy" with the non-steeped leaves.

Some people might say they last longer, too. Your call. In the interest of providing the most conservative advice for the greatest majority of hobbyists, I stand by my recommendations to employ some form of prep, as outlined here.

As far as "placement" and "depth of litter bed" is concerned, that's really up to you. I've gone over the possible issues with adding a proportionately large influx of fresh leaves and botanicals to an established aquarium at once, and I stand by my recommendation to go slowly. As you are aware, rapidly adding a bunch of leaves that will contribute to the bioload of the aquarium, not to mention, potentially decrease the pH, can have some serious consequences for the animals in your system.

Besides, part of the fun is watching the aquarium "evolve" over time. Test pH, ammonia, and nitrite regularly during the first few days after you've added the botanicals to an existing tank, and perhaps pH and nitrate/phosphate on the longer term, to establish "baseline" parameters and monitor any trends as your system matures. "Test, then tweak" is a favorite old aquarium adage of mine for a good reason.


Depth-wise, it's your call, and wide open for experimentation. In a properly filtered, well-maintained aquarium, I see little reason why you couldn't create a very deep litter bed, approaching 8-10 inches (20.32-25.4 cm) deep- or more! In nature, leaf litter beds may be several meters deep!

Now, I realize that an aquarium is not an open-system like a stream, and that there are upper limits to what you can do, so the real takeaway here is that, with careful experimentation, observation, and a willingness to make "mid-course corrections", you as the hobbyist can try all sorts of things with regards to depth and composition of your leaf litter bed.

Bonus questions?

Hmm, how about what to do about the moldy spots you sometimes see on leaves? This is particularly common with Catappa, Jackfruit, and Magnolia. It happens somewhere along the line, during the chain of preparation by the collectors, packaging, and transport. Magnolia, in particular tends to have more surface mold than most other leaves, in my experience, and I believe it's because the waxy cutin layer on these leaves retains more moisture than other leaves, and in a sealed plastic bag, they tend to release it and the resulting moist "environment" in the bag tends to recruit some mold on the surfaces of the leaves.

I've never, ever had any problems by using leaves which have had mold on them in my aquariums. Because I take the time to prepare my leaves before using them, it's always been a "non-issue" in my experience. Just use some common sense. If the leaf is just covered in scuzzy mold, dump it. With a little "dusting" of mold, clean it and prepare as recommended before using in your aquarium.

A lot of hobbyists ask us which leaf they should use to simulate a habitat from such-and-such a locale. It's a logical question, and one which super hardcore biotope enthusiasts might rightly grapple with. My answer? Since most of the leaves we offer are found in multiple locations around the world, and some are not even tropical (like Magnolia leaves and Live Oak leaves), the best we're doing is creating a reasonable facsimile of what's found in say, the Borneo Jungle or Amazonian rain forest.

Rest assured, I'm constantly working with  my suppliers around the world for more varieties of leaves, particularly those from some exotic locales, and we'll have them really soon. Sustainable, responsible collection of naturally fallen leaves, properly prepared for our use, is the driving factor. We'll keep at this. However, for now, just understand that the leaves you have available will work just fine.

In fact, I don't think your fishes will notice one bit that they're swimming over leaves from a completely different continent. And trust me, once the water "tints" and the leaves start breaking down, unless you're really uptight, you won't be all that concerned about your Amazonian-themed tank having an Indian Jackfruit leaf in it.

Perhaps in the future, we will recognize some specific microfauna and such from leaves found in very specific locales, and how they impact the aquatic environment and its inhabitants, but for now, we're content (?) to replicate it on a more "superficial" level. 

Okay,  in this "magnum opus", I think we've just about covered the initial ins-and-outs of working with leaves in your aquarium. Sure, we could probably go on and on and on about water chemistry impact, tuning your aquarium for optimal performance with leaf litter, husbandry, etc., etc., but I think this is a good start.

Our community's collective experiences and subsequent discussions will only add to the growing body of knowledge about the concept, aesthetics, function, husbandry, and long-term dynamics of the botanical-stye, blackwater aquarium.

That's where YOU come in!

Keep experimenting!

Stay fascinated. Stay excited. Stay curious. Stay disciplined. Stay adventurous. Stay observant...

And Stay Wet.

Scott Fellman

Tannin Aquatics 


Scott Fellman
Scott Fellman


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