Blackwater in theory and in practice...

The term "blackwater" is becoming super-common in the aquarium world these days, isn't it?

The idea of blackwater aquariums, with their tinted color and mysterious aesthetic is hardly "new" to the hobby world. No one really "invented" this. No one was the person who said, "We should all throw leaves and seed pods in our tanks..." It just sort of...evolved.

Yep. And evolution is what we're going though now; at scale.

We're seeing hobbyists going beyond yesterday's "blackwater tanks look dirty" mindset, and embracing the aesthetic for what it is: A very natural-appearing "vibe" that replicates conditions found in certain natural environments around the world.

And with this acceptance of the "look" and ephemeral nature of botanicals in aquariums, a definite "mental shift" has occurred.  This to me is most significant and important. Many hobbyists who have previously bought into the prevailing "brown is dirty" mindset are giving blackwater, botanical-influenced tanks a try, rather than flat-out dismissing the idea and (in our opinion) antiquated notions pushed around on the web that these aquariums are difficult to manage, unstable, and otherwise simply "fringe" novelties, rather than a legitimate specialty within the hobby.

We're also seeing a growing body of science-backed evidence that humic substances, a key component of "blackwater" have significant health benefits for fishes, and may be among the most important factors which contribute to their health in both the wild and in captivity.

This revelation backs up what many aquarists who dabbled with catappa leaves and bark and other stuff in botanical-influenced aquariums, particularly Betta breeders in Southeast Asia, have asserted for years. In particular, it's thought that these compounds, derived from botanicals, have anti-fungal and anti-parastic properties, and offer protection against oxidative DNA damage and from physiological stressors. With these health benefits now more clearly understood, there are more reasons than ever to appreciate the role that an environment which accumulates these humic substances can play in overall fish health.

Seemingly out of nowhere, the idea of creating a deep, dark, mysterious blackwater aquarium utilizing botanical materials has become a sort of “thing.” Now, in all fairness, hobbyists have been experimenting with blackwater aquariums for decades.

The whole concept of utilizing these materials to create not only healthy  environments for ort fishes, but to create aesthetically fascinating, remarkably faithful functional replications of wild habitats is being given some new life. The idea of blackwater aquariums being seen as a "side show" curiosity is falling by the wayside, as hobbyists are utilizing these types of tanks to keep even fishes which have been with us for decades, and achieving remarkable results...and discovering a new aesthetic and enjoyment in the process. 

And, for almost as long as hobbyists have been playing around with them, there has been confusion, fear, misunderstanding, and downright misinformation on almost every aspect of them! We’re still seeing a lot of that confusion. It’s important to really understand the most simple of questions- like, what exactly is “blackwater”, anyways?

A scientist 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 SiO2 contents. Tannins are 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 “black-water.” 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.

In summary, natural black waters typically arise from highly leached tropical environments where most of the soluble elements 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 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, 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.

Interestingly, however, these waters are surprisingly low in dissolved organic compounds (DOC). In fact, Rio Negro black waters are theorized to have low DOC concentrations because of thdiluting effect of significant amounts of rainfall, and because they are diluted by clear waters from nearby systems low in dissolved organic compounds.

Sort of  self-regulating, to an extent, right?

In the podzolic soil where blackwater originates, 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!

As aquarists, we tend to over-emphasize the role of leaves and botanicals in creating “blackwater”, because we approach things a bit differently than Nature does.  In an aquarium, we can use a slightly different technique to achieve the same results as Nature. It starts by utilizing water with little to no carbonate hardness (that’s sort of equivalent to what you get in Nature when the water drains over those weathered rocks and soils).

That’s why you will hear us constantly recommend that you purchase a reverse osmosis/deionization (RO/DI) unit to prepare water. The product water produced by an RO/DI unit is more “malleable” to creating lower pH blackwater with a few different approaches. 


We impart color-producing tannins into the water in our aquariums by utilizing leaves and other botanical materials, like seed pods, cones, bark, and even wood. Confusingly, you can achieve the look of blackwater habitats even with relatively hard, alkaline water. Of course, there is more than just the aesthetics, right? Many of these materials will also impart complex compounds, like polyphenols, polysaccharides, lignin, and other substances into the water as well, which can have positive influences on fish health, and the overall aquarium environment. 


So, the approach to create “aquarium quality” blackwater is surprisingly simple, really. Start with high quality RO/DO water, add some botanical materials like leaves, bark or seed pods, and in theory, you’ve created the aquarium equivalent of “blackwater.”  I mean, it’s not quite that simple, as the easy process belies the complex chemical interactions that take place in the water to create these conditions, but for most of us, that’s kind of how it works on a superficial level.


We've talked a lot about the many cautions and even "myths" surrounding keeping fishes in low-pH environments. We've learned that by simply not being afraid because "they" have made them seem so scary and unmanageable for years. Rather, we're revisiting these parameters and trying to learn exactly what happens. We're starting to discover that the low pH aquarium is entirely functional, if one learns the dynamics. Much like the previous generations' discoveries about the aquarium "functionality" of African Rift Lake habitats and coral reefs, we're discovering that these are simply different types of environments which can be replicated and managed long term in the aquarium.

Our understanding of the nitrogen cycle and the toxicity of ammonia versus ammonium, and the importance of "stability within a range" is starting to yield some results. I firmly believe that the next few years will bring about significant change-and even breakthroughs- in the way we as a hobby manage, care for, and spawn fishes such as Altum Angelfish, which have long been though "problematic" and "difficult" because of their specialized needs.

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. 

It's not only vital for us to understand how these habitats work in Nature- it's important for us to be able to replicate some of its functions if we want to be able to keep and breed the fishes that we keep which hail from these habitats.

It's as much about accepting a different way of thinking as it is about actually learning what's going on and attempting to replicate the function of these unique habitats.  It's always been there for us to examine...we've just been approaching it with a jaded mindset. Now, we're looking at them for what they are, the benefits they provide our fishes, and just how to replicate them properly in the aquarium.

The key takeaway here is to not simply accept everything you read about this type of aquarium (even in our blog!) without giving it a more detailed look yourself, and consulting with those of us who have a lot of personal experience with them. A healthy dose of open-mindedness, coupled with some knowledge and skepticism go a long way towards success! 

Obviously, we can't cover every detail about every misunderstood aspect of the blackwater, botanical-style aquarium in the scope of this blog piece/podcast. Those of you who visit our web site frequently know that we literally have hundreds of articles on these topics, many of which comprise a sort of "living document" and demonstrate the evolution of the practices that we use and the experiences that we accumulated with this unique hobby niche. 

We're privileged to have a front-row seat to this evolving hobby speciality (okay, you can call it a movement!), and most important, are honored to be a part of the growing global community of fascinating, creative, courageous, and engaged hobbyists who are forging a dynamic new path in this amazing hobby that we all love so much.

Stay studious. Stay curious, Stay open-minded. Stay bold...

And Stay Wet.


Scott Fellman

Tannin Aquatics 

Scott Fellman
Scott Fellman


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