School of rocks...Or, how geology influences blackwater habitats (in the wild AND the aquarium!)

As we delve deeper into the world of 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 "cliche' it" and say that blackwater is water "...which has a low pH caused by dissolved organic materials and looks the color of tea." 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.

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


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 limited, but I'll convey what I DO know to you here...

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 near 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 lousy 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 pretty much any Amazonian aquatic habitat! 

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 full and humic substances- poor in nutrients and electrolytes. It's characterized by having sodium as one of it's 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 story short, the physical charachteristics 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.

Now, 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 lot more interested in the "decaying vegetation" (you know, the leaves, twigs, seed pods...stuff like that!) which influences the waters. And, if we're really into creating realistic substrates, can't help but think that we'd have to source a podzol-type material to use as the base...And I am not aware of a commercial product that is podzol based which is available in the hobby as an aquarium substrate (entrepreneurs- here's one for you!)...Shit, I just give away some of my best ideas, huh?

Until such time as a substrate like that becomes available, I'll keep using the available aquarium substrates which don't impact pH and alkalinity as the literal "base" for my blackwater aquariums. 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 or whatever to do the job in your low pH and alkalinity blackwater aquarium, right?

And then there is that question about utilizing rocks in your "igapo" aquascape...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, 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 toss some in. 

I personally, of course, would be a bit more forgiving, but you won't find rocks in my igapo tank!


So much to consider in our tinted world, isn't there? Especially when it comes to rocks, sand, and the stuff it influences...

A little "deep-diving"into these seemingly obscure topics can sometimes give us some clues that can influence our aquarium practice.

Stay curious. Stay diligent. Stay informed. Stay inspired...

And Stay Wet.


Scott Fellman

Tannin Aquatics 




Scott Fellman
Scott Fellman


Leave a comment