Deep dives, black water, and rocks...

The beauty of the aquarium hobby is that we can find no shortage of inspiration from the natural world.  You don't have to look all that hard to find it.

And, if we look really hard, we can find combinations of various, seemingly incongruent components that we might not have ever considered using together before. And we can apply them to our aquarium practice and be entirely consistent with Nature!

As we delve deeper into the world of botanical-style, blackwater aquariums, I think it becomes more and more important for us to understand the wild blackwater habitats of the world.

Specifically, how they form, and what their physical characteristics are. It's easy for us to just go the  "cliche' route" and say that blackwater is water, "...which has a low pH caused by dissolved organic materials and looks the color of tea."  You could just leave it at that. You know, the standard line used for decades.

Not untrue, but not really all that helpful in understanding exactly what it is, IMHO.


And more important, understanding why it has these characteristics.

And there are some things which contribute to the overall habitat of blackwater environments- specifically, how they form.

Well, it starts with the study of rocks...Yeah, Geology.

Hey, don't start yawning on me...

I should first start of by freely admitting that I sort of- well, dozed through the limited number of geology classes I took in high school and college, and never knew that the time I spent in those classes drawing pictures on the back of my notebooks would ever come back to haunt me decades later, when I'd have to re-familiarize myself with all of this stuff!

So, my understanding is admittedly quite limited, but I'll convey what I DO know to you here...And just how it relates to our area of interest.

Blackwaters in areas like Amazonia (one of our fave locales, of course!) drain from an area known to geologists as the "Precambrian Guiana Shield", which is comprised of sediments include quartz, sandstone, shales, and conglomerates, stemming from the formation of the earth some 4.6 billion years ago. As a result of lots of geological activity over the eons, a soil type, consisting of whitish sands called podzol is formed.

Podzols typically derive from quartz-rich sands, sandstone, and other sedimentary materials in areas of high precipitation. (Hmm, like The Amazon!). Typically, Podzols are kind of well, shitty for growing stuff, because they are sandy, have little moisture, and even less nutrients!  

A process called podzolization (of course, right? WTF else would you call it?) occurs where decomposition of organic matter is inhibited. Numerous microbes and plants consume some of the nitrogen, and while eaten by other organisms, convey what's left to the even lower-lying forest habitats.

The Amazonian blackwater rivers are largely depleted in nutrients, having passed through the lowland forest soils as groundwater, from which weathering has already occurred. As a result, layers of acidic organics build up. With these rather acidic conditions, a deficiency of nutrients further slows down the decomposition of organics. So, yeah- lousy soil for growing stuff...But guess, what? They form the basis of the substrate in many Amazonian aquatic habitats! 

And the water which flows over this soil is what we call "blackwater",  which achieves it's unique color from a really high content of dissolved  humic substances- poor in nutrients and electrolytes. It's characterized by having sodium as one of its major cations (ions with fewer electrons than protons, giving them a positive charge), which means it has low alkalinity. Typically, the pH and electrical conductivity values are less than 5.0 and 25 μS cm–1, respectively (pretty freakin' low!).

So, to make a very long and intimidating story short, the physical characteristics of blackwater habitats are influenced as much by the geology as anything else!  

That is to say, all of the dissolved humic substances which give these bodies of water their unique look are "enabled" by the geological properties of the region. And from the "trace element perspective (the reefer in me), only Fe, B, Sr, Pb and Se present consistent concentration variabilities to influence the chemistry of these waters...Like, this water has very low concentrations of trace elements.

That's why you'll often see simple fine, white silica-type sands on the bottoms of so many Amazonian streams and rivers. They originate up in the mountains and are transported by various means into the lowland areas. I mean, there is way more to this process than I can convey here- but it's a study in the relationship between seemingly unrelated elements and how they come together.

Now, I admit that this is probably more than you will ever care to know about how sand works in your fave blackwater habitats, but I think it's important to understand that it's all kind of...related. In fact, it makes it a lot easier to understand how blackwater systems came to exist and function when you consider this "big picture" stuff!

And of course, we're a hell of a lot more interested in the "decaying vegetation" (you know, the leaves, twigs, seed pods...stuff like that!) which influences the waters.

And the substrates we select for our tanks play an important role in being able to create and maintain such natural conditions.

So, using a quality substrate material which doesn't impact the pH or buffering capacity of the water to any great extent is important...The reality is that just having an awareness of what goes on in the natural aquatic habitats we love gives us a nice "leg up" on this stuff. You're obviously not going to use a strongly buffering substrate like aragonite, calcite, or whatever to do the job in your low-pH-and -alkalinity blackwater aquarium, right?

Of course not. Choose silica.

And then there is that question about utilizing rocks in your  "Rio Negro" habitat or "igapo" aquascape...

Like, why don't you find rocks in these habitats? 

As you know from my long-winded description above, I'm no expert-or even a novice- on geology or geochemistry, or anything in that subject area, for that matter....However, based on my research into this stuff, as related above, it goes without saying that these are hardly conditions under which rocks as we know them could form.

Oh, sure, you might find the random rock in the igapo that was washed down from the Andes or some other high-country locale in these forests, but it's a pretty safe bet that it did not evolve there. This also helps to explain why the blackwater habitats are generally low in inorganic nutrients and minerals, right? 

So...if you're really, really hardcore into replicating an igapo, like, one of those biotope freaks- you'd probably want to exclude rocks- especially if you're entering one of those biotope aquarium contests, astute judges would (rightfully) nail you on scoring for falling back on your natural inclinations as an aquascaper and tossing some in. 

I personally, of course, would likely be a bit more forgiving, but you won't find rocks in my igapo tank! I am not even interested in looking at them. Nope.

Besides, there is something far more compelling and romantic about leaves, seed pods, and wood than there is about a bunch of rock, right?


Okay, don't answer that...

However, you WON'T find any rocks in my "igapo" tanks...


Rather, we choose to concentrate on the more "ephemeral" components of the habitat, and rightfully so!

Our ability to mimic this aspect of the flooded forest habitats is a real source of benefits for the fishes that we keep- and a key to unlocking the secrets to long-term maintenance and husbandry of botanically-influenced aquariums.

The transformation of dry forest floors into aquatic habitats provides a tremendous amount if inspiration AND biological diversity and activity for both the natural environment and our aquariums.

Flood pulses in these habitats easily enable large-scale "transfers" of nutrients and food items between the terrestrial and aquatic environment. This is of huge importance to the ecosystem. 

As we've touched on before, aquatic food webs in the Amazon area (and in other tropical ecosystems) are very strongly influenced by the input of terrestrial materials, and this is really an important point for those of us interested in creating more natural aquatic displays and microcosms for the fishes we wish to keep.

Creating an aquascape utilizing a matrix of leaves, roots, and other materials, is one of my favorite aesthetic interpretations of this habitat...and it happens to be supremely functional as an aquarium, as well! I think it's a "prototype" for many of us to follow, merging looks and function together adeptly and beautifully.

Now, I think at least part of the reason why we're seeing success with utilizing botanicals in our aquariums is that fishes are instinctively "programmed" to utilize many of these materials as both "feeding substrates"- serving to foster the growth of biofilms, fungal growths, and epiphytic algae- and as food items in and of themselves. (Yeah, "pellets and flakes" are NOT part of their natural diet... 😆)

The addition and replenishment of leaves and other botanical materials which we execute in our tanks definitely mimics, at least to some extent, the processes which occur in these habitats which transfer food and nutrients into the aquatic habitat.

And with the ability to provide live foods such as small insects (I'm thinking wingless fruit flies and ants)- and to potentially "cultivate" some worms (Bloodworms, for sure) "in situ"- there are lots of compelling possibilities for creating really comfortable, natural-appearing (and functioning) biotope/biotype aquariums for fishes.

So much to consider in our "tinted world", isn't there? Especially when it comes to rocks, soils, sand, and the stuff they influence- or don't influence, as the case may be...

And I cannot state often enough that we should continue to explore the use of leaves-to create leaf litter "beds" in our aquariums. There is SO MUCH there to gain from this it's not even funny...

Like any environment, leaf litter beds have their own "rhythm", fostering substantial communities of fishes. The dynamic behind this biotope can best be summarized in this interesting excerpt from an academic paper on Blackwater leaf-litter communities by biologist Peter Alan Henderson, that is useful for those of us attempting to replicate these communities in our aquaria:

" 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.”

There is a hell of a lot going on in leaf litter beds. A lot which makes them extremely compelling subjects for aquariums.

So, for all of the above reasons, and many others that we can't touch on in this brief piece- consider trying to replicate a leaf litter bed habitat in your next aquarium! 

And of course, I cannot encourage you enough to try and"deep-dive" just a bit to get more "background" on these topics from scholarly sources. Applying some of the ideas and information to aquarium practice is justifiably fascinating, compelling, and rewarding.

Oh, but what about rocks in blackwater tanks?

Well, yeah, you CAN play with rocks in a blackwater aquarium. Nature has a prototype for THAT... You need to study a bit.

In fact, you can have, rocks, leaves, wood and blackwater all together. It's just about context. It's about understanding how and why these materials come together, what factors conspire to cause this, and what can happen when it does.

A little research into these seemingly obscure, and perhaps seemingly unrelated topics can sometimes give us some awesome clues that can influence our aquarium practice in ways that we haven't even imagined. Clues which may lead to further evolutions and improvements to our practices. Clues that can help us continue to create compelling aquatic displays. Clues that will definitely make this amazing hobby all that more enjoyable!

Stay diligent. Stay intrigued. Stay curious. Stay studious. Stay informed. Stay inspired...

And Stay Wet.


Scott Fellman

Tannin Aquatics 

Scott Fellman
Scott Fellman


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