How come there are no rocks in the igapo? (And other nagging questions...)

Yes, some of us have this strange obsession with blackwater, botanical-style aquariums- and have this desire to replicate some of the aesthetics, form, and function of the natural igapo seasonally-inundated forests of Amazonia. (Maybe it's just me...but you're already read on, okay? ) We are often faced with a bunch of questions about how to represent them in aquariums. Today, I address two of the ones I've had for a long time, and judging by the number of inquiries we receive- so have many of you (well, many of you who play with this arcane sort of stuff, anyways!)...And, like so many of the topics we discuss here, the "answers" are sometimes less than satisfying, often leading to more questions! 

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. 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....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. 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...


What kinds of plants are coming from blackwater habitats? And shouldn't I be able to keep them?

Well, let's tackle the second part of the question first. I mean, I'm no plant expert, and could easily be schooled by even the most novice-level serious aquatic plant geek, but here is my thinking: First off, you're right...some plants should be able to do well in a blackwater aquarium. I mean, in nature, they're often found in extremely low nutrient, low pH environments. Our blackwater aquariums seem to be largely devoid of carbonate hardness (if we're using RO/DI water), and minimal sources of carbon for "fertilizer..."

In these low-pH habitats, CO2 is the real source of inorganic carbon (which plants seem to love, right?), so you'd think that certain plants would love this. And they do, in my experience! Well, perhaps not all of 'em LOVE it, but many "tolerate" it...A hardcore planted-tank person, with a "primary motivation" to create a blackwater planted tank, may not like my approach or attitude, but for those of us who have made the "planted" aspect somewhat lower on the priority scale than other aspects of the tank, it's pretty good... :)

Many Cryptocoryne, for example, come from very low pH, low nutrient habitats (like 4.0-5.5). Based on my personal experience with plants like Myriophyllum, Cabomba, Cryptocoryne, Tonina, Anubias, Polygonum "sp. Kawagoeanum", and others, I'd say that, with good nutritive substrates (like the ones made for planted tanks, or a "dirted" substrate you can create, or even one that's influenced by "botanical mulch" (Cory Hopkins- calling on YOU! ), there is potential for more plants to grow in these systems.

I'm leaning towards those rich substrates...CO2 can be "preserved" with minimal surface agitation, so you might be able to "cheat" a bit and avoid having to inject it.  Light intensity is another part of the equation, and with the dark, tinted water, you need to get some decent lighting in there (LED's are my personal weapon of choice).  

Some known residents of the igapo waters? Well, there are just a few that I was able to locate: Polygonum, Azolla, Pistia, Salvinia, Ceratopterus, and a few other "floaters." 

In my own "ignorance bubble", I make the "coral farmer's analogy', in which I've found over the years that you can compensate somewhat for lower light by providing other parts of the equation (like nutrients, trace elements, and food) and get good growth. Yes, hardly scientific, shockingly speculative...but it worked. And based on my (admittedly limited) experience with plants in blackwater, botanical-style tanks, I'd say it works in these situations, too. Now, I admit, I've never gotten the lush, full look of those clearwater, gnarly high-tech planted tank you see splashed all over social media...but I have gotten some growth, and am willing to sacrifice some of the "epic" growth for love of the entire blackwater aquarium- of which plants are merely a "minor component"  to me (I Know, I just lost the respect of every aquatic plant lover out there- but hey...honesty, right?)

The reality is that many of the (South American) habitats which we play with simply don't have much in the way of true aquatic plants in them. For example, the igapo flooded forests and small streams just don't have much more than epiphytic algae and submerged terrestrial plants in them.  I think it's more of a matter of trying various plants which tend to come from lower ph, blackwater habitats, and applying these ideas to their care. My list is ridiculously superficial, of course- so you hardcore plant people will have to take the flag and run with this!

(The Uakari is the de facto expert on igapo vegetation!)

Now, my other "challenge' to plant lovers in general: Let's figure out which terrestrial plants can tolerate/grow/thrive under submerged or partially submerged (blackwater) conditions. Perhaps a more "realistic" (not in the hardcore "biotope aquarium contest" context, of course) avenue to explore in this regard?

I've got one tree for you to research...the dominant terrestrial plant in this habitat is Eugenia inundata... Don't think I'm not well underway in my (somewhat futile) efforts to see if we can secure fallen leaves of THIS plant! You'll also find Iriartea setigera, Socratea exorrhiza, Mauritiella aculeata palms in these areas.. Like so many things from the Amazon, it's not easy (read that, damn near impossible) to secure botanical material from this region, so the proverbial "Don't hold your breath waiting for this" comes to mind! Oh, and the submerged grasses we see and drool over in those underwater pics from Mike Tuck and Ivan Mikolji of these habitats? They're typically Paspalum repens and Oryza perennis.

Paspalum is found in North America, too...possibly a species you could obtain?

(Paspalum. Image by Keisyoto. Used under GFDL)

Perhaps you could, right? And it's that kind of stuff that keeps us working away.

And the absence of rocks i nthe igapo? Well, yeah, it's a fact...but I like rocks, so &*^%$ it.

Good attitude, right? :)

On a more serious note- I realize that this discussion probably only opens up more questions, and is a little short on "hard facts" to help you make decisions...but it gets the discussion going, and hopefully, stimulates further investigation by many of you more experienced, highly talented aquarists who make up our amazing community!

Keep asking questions. Keep searching for answers.

Stay engaged. Stay excited. Stay curious. Stay creative. Stay resourceful...

And Stay Wet.


Scott Fellman

Tannin Aquatics 





Scott Fellman
Scott Fellman


Leave a comment