Much has been discussed about the proper foods feed our fishes, and lots of anecdotal and other evidence and theories have been bandied about by hobbyists for decades about who eats what and where. It's one of the more interesting and intruiging topics of discussion we have as hobbyists, in my opinion, and there are so many factors which contribute to "who eats what" among the fishes that we keep in our aquariums.
Some of the most interesting food sources for tropical to me involve what scientists call allochthonous input, such as plant materials and terrestrial insects. And even more interesting are organisms like "tree-living sponges", which become part of the food chain during higher water periods of time in the intruiging Amazonian streams we obsess about so much here.
And of course, there's algae.
Now, the interesting thing to me is that the fishes in blackwater, leaf-litter habitats depend little on algae as a primary food-mainly because there's not a whole lot of it in these habitats. They'll eat the stuff when it's available, but because algal densities are lsol ow in this type of habitat, fewer creatures (Pseudopalaemon sp. shrimp are an exception) consume algae as their primary food source.
And, with varying seasonal water levels in these streams, food inputs and fish populations change in both size and composition, creating a sort of "partitioning" of available resources.
Many fish species take food from what are known as "allochthonous sources" (i.e. food originated from sources outside the aquatic habitat), such as insects, other invertebrates, and plant parts that fall from the nearby trees. Like, remember seeing films of Pacus 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 then, of course, there are terrestrial insects, which form a large part of the diet of many fishes.
Yeah, terrestrial insects are a very important and significant part of the diet of some small characins. 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, ants tend to become vulnerable to predation once in the water, so they are "easy pickings" for tetras! The predominance of ants in the gut content analysis 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.
The "allochthonous inputs" of tropical streams are really fascinating to me, for the reason that these are some of the easiest food items in many fishes's diets for us to replicate as naturally as possible. We've discussed before that items like Blood Worms represent an excellent, highly "realistic" representation of the insect larvae that fishes from these habitats consume.
Since items like ants and various flies are such an important component of the diet of many fishes, including things like fruit flies, small houseflies, and the aforementioned small ants in your fishes' diets is actually a really realistic representation of part of what they consume in the wild!
And then there are fungi, biofilms, and detritus...The literal bottom of the food chain; some of the things we as aquarists have traditionally found the least desirable from an aesthetic standpoint to have in our tanks. Ironically, these are some of the most important components of the food web, and are consumed by a wide variety of aquatic life forms that live in these streams as part of their diet. Again, only certain shrimp present in these waters are more likely to consume them exclusively; however, fishes, being somewhat opportunistic, will consume them "as needed."
And gee, don't we have some good "on site production" of biofilms and fungi in our decomposing leaves and botanicals? Yeah, we sure do! We're really good at that. Many fishes will consume these items as a part of their daily "grazing" activities. Now, our aquarium fishes get a bit spoiled, especially after being with us for a while and knowing that they're never more than a few hours away from stuff like brine shrimp and black worms, etc. However, I can't help but imagine if there is some value to abstaining from feeding them prepared foods say, once a week, to let them sort of engage in their natural, instinctive feeding habits, like picking at the substrate, etc. In a botanical-style aquarium, this type of "feeding abstinence" could easily be achieved, right?