What about rock?

How come you don't see rocks in those pics of the igapo inundated forests?

Oh, this is a good one...

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 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 the 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, most low-lying Amazonian forest soils are really low in nutrients. The soils are nutrient-poor, acidic "podzols..." 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 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, and that soil type, consisting of whitish sand we podzol is formed.

However, what little nutrient 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, what little nutrient remains is released into the waters, too. And it's 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 kind of dozed through geology classes in college...much to my regret now. 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 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...

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 the rocks we offer 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...


The fact is, so many of you have asked us to offer new rock types that we simply couldn't not do it. We have a growing customer base fro ma variety of aquatic and aquascaping "disciplines", and helping to foster creativity is what we're all about..and giving you the options to make choices that worse for you!

However, 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.

If you're faithfully trying to recreate a highly acid, soft water habitat devoid of rocks -than you'd likely want to avoid using rocks of any kind to a great extent. Right? 


And substrates? Well..

In fact, you'd seek out a podzol-type material to use as the base...And I am not aware of a commercial product that is podzol -ased which is available in the hobby as an aquarium substrate (entrepreneurs- here's one for you!)...


Research with planted substrates is totally the way to go at this time, IMHO...

In the mean time,  I'll keep using the available aquarium substrates which don't impact pH and alkalinity as the literal "base" for most of 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?


So, researching and experimenting is the best I can tell you right now. 

Oddly unsatisfying for the "I want an answer for everything-now" crowd- but that's where we're at in the hobby at the moment. 

Geologists and geochemists wanted! 

Until next time...

Rock if you want to. Avoid if you feel it's appropriate.

Stay bold. Stay curious. Stay experimental. Stay studious...

And Stay Wet.


Scott Fellman

Tannin Aquatics 

Scott Fellman
Scott Fellman


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