Amateur geology, sands, rocks, and blackwater-and putting it all together.

Hey Scott- how come you don't see rocks in those pics of the igapo inundated forests? Can I Include them in my "urban Igapo?"

Oh, this is a good one. A question we get all the time.  First off- you CAN do whatever the hell you want. However, if you're trying to faithfully represent one of these habitats in your own tank, you should read on bit...


We've talked about this before, but I think it's a story worth telling again! It's story of minerals, sediments, soils, and the processes which impact their formation.

And it starts way up in the mountains...

The "whitewater" rivers rush quickly down from the mountains of Peru and Bolivia, too rapidly for clay and silt to be stripped from them. The rocks from these mountainous areas offer minerals and nutrients such as nitrogen, attached to the silt and clay, and minerals like illite, montmorillomite (hey, we know that one from shrimp geeks!), and chlorite, to nourish the lower-lying areas. In these lower areas, 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 these lowland forest soils as groundwater, from which weathering has already occurred. "Hydro-geomorphic processes" ( i.e.; a fancy way of referring to part of the stuff that makes rocks!) are far less intense than they are in the upland, mountainous regions, with their abundance of minerals, nutrients, slits, and sediments.

In other words, rocks form high in the mountains in these locales. Most low-lying Amazonian forest soils are really low in nutrients. The soils are nutrient-poor, acidic types. It's been suggested that most of the available nutrients are taken up by the root mats of the dense plant growth in these forested areas. And even the rainwater provides little in the way of nutrient for the plants which grow there.

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 including 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 nutrient- depleted soil type, consisting of whitish sand we call podzol is formed.

What little nutrient is there is typically returns to the soils by means of leaf drop from the trees which grow there. And of course, when the water returns to the forest floors, residual nutrients are released into the waters, too. And they're quickly utilized by the resident microorganisms. Serious nutrient cycling, right?

I'm no expert-or even a novice- on geology or geochemistry, or anything in that subject area, for that matter....I admit that I kind of dozed through geology classes in college...much to my regret now! (Hot tip to young fish geeks: Don't nap in Geology or Meteorology might need them some day!) However, based on my research into this stuff, it goes without saying that these are hardly conditions under which rocks as we know them could form.

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 most certainly 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...

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

Well, it goes back to Geology. Again.

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


Let's go back to those podzols one more time, okay?

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

A process called podzolization (Are you fucking kidding me? Well, what else would you call it, right?) 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.

Same story as above- 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 crops...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 and fulvic acids- it's 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! Tossing a bunch of Alder cones into your tap water does NOT create "blackwater!"  Just get that shit out of your head once and for all, okay?

When I was formulating our "NatureBase" line of substrates, I spent a lot of time studying the geology of the regions which led to the formation of these podzolic soils, to see if I could create something that had similar characteristics. Unfortunately, you just can't roll up to the local garden center, ask for "podzolic soils", and pick up a 50 pound bag! (Trust me, I tried).

So I made my own


Yes, suffice it to say, the soils and rocks have a profound influence on the formation of blackwater.

And 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 sufficient 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 meaningly convey here- but, suffice into say,  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 geology influences your fave blackwater habitats, and I'm all over the place as usual- 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, right?

Well, yeah. 


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

Substrate is pretty important, IMHO.

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, and such to do the job in your low-pH-and -alkalinity blackwater aquarium, right?

Of course not. Choose silica. Or  commercially available silica-based substrates which won't impact pH and hardness. 

So, back to that question about utilizing rocks in your  "Rio Negro" habitat or "igapo" aquascape...


If you're really, really hardcore into replicating an igapo, like, one of those biotope freaks- or even, like me- 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, which is why I'd be a completely shitty judge- but you won't find rocks in my igapo tanks! 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...

Don't cry, rock lovers...

Yet, there are plenty of Amazonian and other habitats with tinted water and rocks.

And in the aquarium, we have many options to faithfully recreate, or simply gain inspiration from -these habitats. It's okay to use rock. Really. 

I mean, it provides a unique and satisfying aesthetic experience for our aquariums, while providing a nice contrast with wood and botanicals. 

Sure, the fact is, some rock will impact the chemistry of your water, and if you're really hardcore about it, you'll have to do some experimentation. I have played with many types of "aquairum available" rocks in my tanks, and I can say that they will typically impact pH and GH a bit; however, to what extent is subject to many variables, ranging from the type of water you start with to the substrate you use, etc. Making generalizations is tricky and "outside of my pay grade" as they say...SO...


As I always say, it's important to understand that we should not specifically limit ourselves to any one rigid way of thinking...We simply have to understand that rocks- like botanicals or wood or anything else we add to our aquariums- impact the environmental characteristics of our closed systems. And, that if we're trying to replicate habitats which don't typically have rocks in them, then to exclude them from our tanks makes more sense, right?

From water chemistry aspect, if you're faithfully trying to recreate a highly acid, soft water habitat -than you'd likely want to avoid using rocks of any kind to a great extent.


Okay, enough off this stuff already. I'm longing to talk more about something more, leaves and stuff!

Stay curious. Stay resourceful. Stay observant. Stay creative...

And Stay Wet.


Scott Fellman

Tannin Aquatics 

Scott Fellman
Scott Fellman


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