A healthy obsession with leaves.

Oh, man, here he goes again....


We have a lot of discussions and inquiries around here about the way blackwater habitats work, and how the physical characteristics and water chemistry found in them affects the fishes which reside there. And funny, we're actually starting to hear definite opinions in the hobby about the characteristics of blackwater, and what constitutes such in the aquarium.

And of course, once we get into "definitions"- specifically those preferred by hobbyists (us), I start getting that old nauseous feeling again- like, "...Here we go, f ---ing this up again with our attitudes..."

Why? Well, because I think we tend to arrive at a conclusion and dig in, based on some narrow, or opinionated points of view. I've been told generalized, almost dogmatic-sounding statements, such as "You can't have a blackwater aquarium unless the pH is below 6..." 

And that's where I get a bit uncomfortable. Opinions are important. But going beyond hobby forums and digging into scholarly research on the topic (of which there are tons for those who would but take the time to search for them and read them) yields some really interesting data that we could work with!

I mean, there are characteristics which science uses to classify blackwater habitats- and pH is one factor. No argument there. However, it's not JUST pH which defines them, and you also have a surprisingly large number of documented blackwater systems with pH levels in "the sixes"- so such stubborn views on what defines these habitat-and aquariums which attempt to replicate them- are a bit generalized, IMHO.



Yeah, based on field studies, you can have a 6.2-6.6 pH, low dissolved mineral content, tinted water, etc. in your aquairum, and still comfortably call it a "blackwater aquarium."


Besides the color and pH, of course, there are other defining characteristics of blackwater systems agreed upon by scientists and ecologists, such as  low electrical conductivity, and negligible dissolved minerals, such as  Ca, Mg, K, and Na. In one study I read, conducted on a number of Rio Negro tributaries, these characteristics were pretty much consistent throughout the region. Most of the habitats in the study were "acid black water streams" with high concentrations of humic and fulvic acids derived from podzolic soils and accumulations of leaves within these systems.

And, with these low amounts of dissolved minerals and corresponding low pH levels come unique challenges for the animals who reside in these systems. And, if you're going to go the route of targeting pH levels in the "fives" (or lower) in your aquarium-a legitimate goal, of course- it's important to consider how to provide "collateral conditions" to keep your fishes healthy.

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 blackwater actually enable the fishes to thrive in this highly acidic water which is devoid of much mineral content typically needed for fishes to survive!

So, again- for those who seek extremely low (by hobby standards) pH levels for their blackwater aquariums, I think it would be a legitimate practice to incorporate leaves and other botanical materials in them for both aesthetics and long-term health benefits to our aquarium fishes residing in such conditions.

Oh, and this intriguing  finding in a study on humic substances in ornamental fish aquaculture really perked up my senses a bit:  "Humic substances are not real alternatives to strong traditional therapeutics. However, they show different advantages in repairing secondary, stress induced damages in fish."

A lot there. Some of which we can't truly "quantify" for aquarium practice in a neat and tidy way. 

So, how do we easily impart some of those "protective" humic substances into the water? Likely by utilizing those leaves and botanicals, right? 

Yeah, I think so. 

And this goes far beyond just the cool aesthetics they impart, too! Like, what we are doing with leaves, twigs, substrate materials, seed pods, etc. has a physiological impact on the fishes we keep. Of course, "how much" and "which ones" is the "art" part of this. Quite honestly, we simply don't have all of the answers. We have to experiment. We have to try and evaluate and go from there.

Yeah, it's an ongoing evolution. However, by re-training our minds to appreciate these natural habitats, the way the look, and more important- how they function- we can create truly amazing "functionally aesthetic" aquariums for our fishes. 

What a powerful incentive to study and enjoy both the aesthetics and environmental enhancement capabilities of the botanical materials we tend to be so obsessed with around here!

A lot to unpack, I know. Where do we start?

Well, my botanical/blackwater obsession started with leaves...

I'm very happy to see so many hobbyists embracing the blackwater, botanical-style aquarium. Not just because, hey- like, I have company that sells stuff for it- but because it's an area of the hobby that was really relegated to "novelty" status for so long, with the mainstream aquarium hobby not really seeing much value in it beyond a "that's different..." mindset.


Now, we're seeing a lot of research by hobbyists into the best way to incorporate botanicals into their aquariums. Many of us are playing with the idea of incorporating leaf litter- something that was given little more than a passing bit of attention a few years ago, if that- into our tanks. This increased level of attention to this environmental niche among hobbyists is reaping benefits for those who have played with it.

Leaves are sort of the "gateway drug", if you will, into our world.


In nature, as we've discussed many times-leaf litter zones comprise one of the richest and most diverse biotopes in the tropical aquatic ecosystem, yet until recently, they have seldom been 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 lack of real understanding about what this biotope is all about- not to mention, the understanding of the practicality of creating one in the aquarium.

Long-held fears and concerns, such as overwhelming our systems with biological materials, and the overall "look" of decomposing leaves and botanicals in our tanks, have understandably led to this idea being relegated to "sideshow status" for many years. It's only been recently that we've started looking at them more objectively as ecological niches worth replicating in aquariums.

The thought behind this habitat can best be summarized in this interesting except from an academic paper on Amazonian Blackwater leaf-litter communities by biologist Peter Alan Henderson, one 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 in our tanks, 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. 

A biotope that deserves your attention and study, indeed.

I encourage every "tinter" to experiment with a leaf litter-zone themed aquarium at some point! You’ll be surprised how far you can take the biotope concept, especially if you strive to be completely true to the niche you’re modeling and aspire to only keep animals found in that niche!

It is known by science that the leaf litter and the community of aquatic animals that it hosts is, according to one study, "... of great importance in assimilating energy from forest primary production into the blackwater aquatic system." 

It also functions as a means to preserve the nutrients that would be lost to the forests which would inevitably occur if all the material which fell into the streams was simply washed downstream. The fishes, crustaceans, and insects that live in the leaf litter and feed on the fungi, detritus, and decomposing leaves themselves are very important to the overall habitat.

In the properly-constructed and managed botanical-style aquarium, I believe that leaf litter and botanicals certainly perform a similar role in helping to sequester these materials. This is an exciting field of study for our community!

As we've talked about before briefly, another interesting thing about leaf litter beds is that they actually have "structure" and even longevity. In several studies I read on the subject, the accumulations of leaves in various streams are documented to have existed in the same locations for years- to the point where scientists actually have studied the same ones for extended periods of time.

Some litter beds form in what stream ecologists call "meanders", which are stream structures that form when moving water in a stream erodes the outer banks and widens its "valley", and the inner part of the river has less energy and deposits silt- or in our instance, leaves.

There is a whole, fascinating science to river and stream structure, and with so many implications for understanding how these structures and mechanisms affect fish population, occurrence, behavior, and ecology, it's well worth studying for aquarium interpretation!  

Oh, did you get that part where I mentioned that the lower-energy parts of the water courses tend to accumulate leaves and sediments and stuff?

It's logical, right?

And it's also interesting, because, as we know, fishes and their food items tend to aggregate in these areas, and embracing the "theme" of a litter/botanical bed or even wood placement,  in the context of a stream structure in the aquarium is kind of cool!

Yes, I could go on and on and on... I literally have an obsession with leaves I the aquarium...a healthy one, for sure! 

And I will, in future installments of this blog, as we learn more and more and accumulate an increasingly large body of work to add to our knowledge base on the topic.

Everyone's input is welcome. It's still very much "ground floor" here. I think it's still way too early to make authoritative statements and to draw stubborn conclusions about most aspects of our natural, botanical-style aquarium "practice."

Keep an open mind. Make appropriate mental shifts. Experiment freely and responsibly. Share your successes, failures, and everything in between with our fellow hobbyists.

Tint the world. Blur the lines. Dream in water.

Stay bold. Stay inquisitive. Stay diligent. Stay resourceful. Stay creative. Stay engaged...

And Stay Wet.


Scott Fellman

Tannin Aquatics 




Scott Fellman
Scott Fellman


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