The intersection of soil, water...and fishes

We had a really interesting discussions on Facebook and elsewhere within the blackwater community, and I must say, they've been incredibly enlightening and inspiring!  

One of the subjects we've talked about before is the idea of utilizing more natural substrate materials, like clays and such, as opposed to more traditional gravels and sands. I often ponder which types would be interesting to use with botanicals to create rich and productive aquatic environments.

I've been playing with a lot of "mud" and "mucky" substrate materials (including decomposing leaves, of course) while sprouting mangroves for my brackish water tank, and it renewed my interest in this topic! I think there is a lot to contemplate.

It got me thinking, not only about the types of substrates that make sense to experiment with, but thinking about the interactions between land and water that occur all over the world- stuff we don't think all that much about as hobbyists; stuff that has profound influence on our fishes, however!

In regards to the substrate materials themselves, I'm fascinated by the different types of soils or substrate materials which occur in blackwater systems and their clearwater counterparts, and how they influence the aquatic environment. Keep in mind that many of the habitats we obsess over, like Amazonian "igapos" and "igarapes" are seasonally-inundated forest-floor features, so it goes without saying that the terrestrial soil composition and associated biomass have significant influence on the aquatic environments that emerge during the wet season.

In general, blackwaters originate from sandy soils. High concentrations of humic acids in the water are thought to occur in drainages with what scientists call "podzol" sandy soils. "Podzol" is a soil classification which describes  an infertile acidic soil having an "ashlike" subsurface layer from which minerals have been leached. That last part is interesting, and helps explain in part the absence of minerals in blackwater. And more than one hobbyist I know has played with the concept of "dirted" planted tanks, using terrestrial soils...hmmm.

Also interesting to note is that fact that soluble humic acids are adsorbed by clay minerals in what are known as "oxisol" soils, resulting in clear waters."Oxisol" soils are often classified as "laterite" soils, which some who grow plants are familiar with, known for their richness in iron and aluminum oxides. I'm no chemist, or even a planted tank geek..but aren't those important elements for aquatic plants? 



In "iagapos "(those seasonally flooded forest areas which lead to blackwater environments), the soils are conducive to good terrestrial plant growth. Interestingly, seed dispersal by fish (a process known technically as "ichthyochory") is thought to play an important role in the maintenance of the diversity of trees in these seasonally inundated forests along the main rivers of the Amazon. 

Thus, aquatic life influences the land! 


Fishes which consume matter found in the substrate (detritivores) and other materials in the substrate (omnivores) also play a fundamental role in the transportation of organic carbon, which is a source of energy for downstream fish communities. Through their foraging activities, these fishes enhance the "downstream transport" and processing of organic material and ensure the proper functioning of the aquatic system and its biological community.

They literally help "maintain" the aquatic environment!

Interesting role for fishes, isn't it? What can we learn from them and their impact on the aquatic environment? Is any of their activity relevant to aquariums?

The whole picture here is kind of interesting to me. Now, granted, it's early in the morning here in L.A., and I might have had a glass or two of wine last night when I started writing some of this stuff down, but I think there is a lot of potentially useful stuff to absorb here!

I mean, we have the terrestrial environment influencing the aquatic environment, and fishes that live in the aquatic environment influencing the terrestrial environment! This is really complicated stuff- and interesting! And the idea that terrestrial environments and materials influence aquatic ones- and vice-versa- is compelling and could be an interesting area to contemplate for us hobbyists! 

It already is, to some extent, as the whole idea of utilizing botanicals (from terrestrial sources) in our aquariums encompasses these processes.

And speaking of terrestrial influences on our fishes- this is kind of interesting:

Many fish species take food from what are known as "allochthonous sources" (i.e. food originating from sources outside the aquatic habitat), such as insects, other invertebrates, and plant parts that fall from the nearby trees.

Like, remember Pacu chowing on fruits that fall in the water? I've even seen pics of Arowanna leaping out of the water to pluck a frog off of a branch! And gut-content analysis of many fishes in these flooded forest areas demonstrates that terrestrial insects form a huge part of the diet of many fishes.

Yeah, terrestrial insects are a very important. In fact, a study of some Hemmigramus species indicated that a whopping 96% of their stomach contents were terrestrial insects, mainly...ants!  

This is actually not surprising, when you think about it, because ants are ridiculously abundant in tropical forests, and in particular in the central Amazon basin, where scientific surveys have estimated that they may constitute as much as three-quarters of the biomass of the soil fauna!


In addition to providing a potentially rich source of energy for Characins and other fishes, ants tend to become vulnerable to predation once in the water, so they are "easy pickings" for tetras! The predominance of ants in the wild diets of Hemmigramus, Hypessobrycon, and other tetras may also indicate that these species feed naturally on the surface of the water, given that these insects tend to float and flail away on the surface after falling into the water.

So, a possible takeaway here is that live ants, those pesky nuisances we all revile- might just prove to be one of the easiest-to-provide, most natural food sources for our small fishes. This is really interesting to me. Like, I'm thinking of grabbing one of those "Ant Farm" kits you get when you're a kid, and using it as a live food culture, lol!

(Hey, this is the kind of stuff real fish geeks do, right?)

In all seriousness, this brief and admittedly very superficial look at the complex relationship between terrestrial and aquatic environments is another area that we could put a lot more "hands-on" research into as hobbyists, particularly those of us who are fascinated by botanical/blackwater habitats. It might just be another case where the answers to some of our questions and problems are literally right in front of our eyes, and we simply have to look at the world around us in proper context, thinking way outside the box. 

It's fascinating to contemplate the things that happen where soil, water, and fishes meet...

Damn, this shit's cool!

Lots to learn. Lots to think about. A lot to experiment with!

Stay open minded. Stay curious. Stay engaged. 

Stay Wet. (and dry, apparently!)


Scott Fellman

Tannin Aquatics

Scott Fellman
Scott Fellman


2 Responses

Scott Fellman
Scott Fellman

March 09, 2018

LOL, great story! Yes, “size and aggressiveness” of the ants is definitely a big consideration! Not being an insect expert, I’d recommend some experimentation here! I’ve not experienced the same thing you have, but I can see how that would be an issue, particularly with really small fishes with correspondingly small mouths!



March 08, 2018

I tried feeding ants. I found a tiny one outside and dropped it in the tank. One of the ember tetras darted up and grabbed it at once. That was as far as the experiment worked. It didn’t manage to swallow the ant as the ant fought back. The poor fish kept trying to swallow it and then shying away from the ant until it abandoned the effort.

If you want to feed live ants find very docile ones.

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