Sifting through science...

A lot of you who are into this blackwater/botanical-style aquarium stuff are sort of like me- looking for some more detailed information than what is commonly available in the hobby literature.

As it turns out, there is some pretty good information out there on this stuff, but you have to ferret it out from scientific papers and other information. In our super-fast, "I need instant concise info now!" age, it's not so easy to find stuff that can spur greater understanding of our beloved topic by simply looking through hobby literature.

It's just not there.


With so many of you now experimenting with blackwater aquariums, the "working knowledge" of our craft is evolving rapidly. Practical applications for botanicals are being discovered, improved upon, and shared daily throughout the world!

Now, I could write blogs simply regurgitating the same watered-down topic materials that's been shared in hobby circles for decades, or I can deep-dive into areas (often well over MY head!) which might provide further insights into just how and why these habitats occur in nature, and what the implications are for aquarists, in the hope that we can glean more from this stuff.

I choose to get in over my head!


That being said, I can present the stuff I've found to be of relevance to our work (at the risk of "watering it down" myself), but, as I repeat ad nauseam, I am not a scientist, so we're going to collectively have to absorb and interpret the bits and pieces we find to see how they can relate to our aquarium obsession.

Now, I'm lucky, because I have a friend who is really into all things science; he literally collects scientific papers and literature on all manner of "stuff", and upon learning of my obsession with blackwater ecology and the desire to find out more, and, over the years, has provided me with enough related, relevant scientific papers on these topics to keep me busy until the sun goes supernova!


So there IS a lot out there on the topic of humic substances and tannin in water, and on the composition of blackwater habitats. It's actually very well studied by science; it simply hasn't "trickled down" to the hobby level to any practical extent (until, oh...maybe...NOW!). And much of it involves understanding the physical environment in which blackwater systems are found in nature.

For example, one of the most important influences on blackwater rivers is the soil and sedimentation of the surrounding areas. It starts with the soils. Blackwater rivers, like the Rio Negro, for example, originate in areas which are characterized by the presence of white sands known as "podzols." (note that, biotope-oriented aquascapers!)

Podzols are soils with whitish-grey color, bleached by organic acids. They typically occur in humid areas like the Rio Negro and in the northern upper Amazon Basin. And the Rio Negro and other blackwater rivers, which drain the pre-Cambrian "Guiana and Brazilian shields" of geology, can in part attribute the dark color of their waters to high concentrations of dissolved humic and fulvic acids! Although they are the most infertile soils in Amazonia, much of the nutrients are extracted from the abundant plant growth that takes place in the very top soil layers, as virtually no plant roots are observed in the mineral soil itself.

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!

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, one of the defining characteristics of blackwater rivers is pH values in the range of 4-5, and low electrical conductivity. 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 i 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!

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

Something in those leaves and botanicals, right?

And this goes far beyond just the cool aesthetics they impart, too! 

Yeah, there is a lot of cool stuff out there. It's just not the easiest thing to dig through.  But there is a lot of information out there if we're up for the challenge.

And we should be up for the challenge!  

I'm reminded of a passage from a speech by the late Neil Armstrong, the first astronaut- the first human- to walk on the moon:

"There are great ideas undiscovered, breakthroughs available to those who can remove one of the truth’s protective layers. There are places to go beyond belief..."

Keep peeling back those protective layers. Keep looking under the stones. Keep digging.

Stay bold. Stay curious. Stay imaginative. Stay earnest. Stay enthusiastic...

And Stay Wet.


Scott Fellman

Tannin Aquatics 


Scott Fellman
Scott Fellman


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