Communities of fishes
Communities of fishes. Our usual mixed community aquarium isn't really much more than a menagerie, a random assemblage of fishes. The first signs that you're moving beyond the "box of fishes on dialysis" phase of "fish-only" fishkeeping are some new questions that start to occur about keeping one species of fish with another, about special requirements fish may have, beyond their temperature ranges and pH preferences. Soon you begin to feel that concerns like "Will my angels eat my neons?" (answer: "Not this week") are resulting in a menagerie rather than a community.
In a menagerie, as one kind of fish after another is added to an aquarium, compatibility questions tend to be phrased as a set of negatives: will they not eat one another? not tear one another's fins? Once in a LFS I noticed a tank with some Carnegiella hatchetfish that were unnaturally close-huddled and swimming in midwater, below their usual surface station. I looked again, and saw, hiding behind a skimpy strand of floating weed, a small alligator gar. This little lurk-and-pounce predator was too small to eat the hatchetfish. Not being in actual danger wasn't enough. They still identified it as a predator. They were clearly as uneasy and stressed as you would be if you were locked overnight in a coat closet with a bobcat.
A less extreme, but also unthinking version of a menagerie is a "Noah's Ark," where fishes live alone or in duos, whatever their natural social needs. Robert T. Ricketts has a good essay about getting beyond the "Noah's Ark" mindset in his article "Schooling fish and their behavior".
Getting beyond the menagerie aquarium. Fishkeepers tend to house their fishes in two basically dissimilar ways, in working fish tanks and in show tanks. It's probably only the "show tank" you'd think to call an "aquarium." Working fish tanks are set up for streamlined upkeep, with a particular result in mind: they can be set up specifically for housing large aggressive individual fish, for encouraging pairs or trios to spawn, for raising young, or for isolation of fishes in quarantine or under medication.
A "community aquarium" is one kind of show aquarium, set up for its decorative qualities and as an opportunity to watch fish in a natural-seeming setting. Too often, even pretty experienced aquarists who are considering setting up a new community aquarium find themselves taking the essentially negative approach I mentioned at the start: how to assemble a group of fishes that are merely "not incompatible." A list gets drawn up of fishes that won't attack one another, won't eat one another, and whose presence won't stress one another. The results generally don't rise above a harmless menagerie, but they will normally lack any ecological significance. Cichlid tanks especially seem to be assembled on the basis of "Can this new one hold its own?" An increasingly tense mosh pit is added to, one new arrival at a time — "Can I fit one more in?" — until finally mayhem breaks out.
By contrast, the community that you might form in an authentic "community tank" should have some ecological cohesion. So an ecological definition of "community" is useful to keep in mind: an association of bacterial, protist, plant and animal populations that are delimited by the particular area they live in and help to shape and maintain its characteristic physical attributes. This definition applies at any scale: there's an aquatic community in the water that collects in a bromeliad in Costa Rica, another one in your planted aquarium, and in a saltmarsh community in great stretches fringing Chesapeake Bay.
If you want to get beyond a menagerie aquarium, it will help to substitute some more positive criteria for adding a fish to the community. More ecological ways of thinking about your community aquarium occupy the articles in the rest of this folder. Then, locating some resources for researching aquarium ecotopes is the theme of the following folder, ecotope aquaria.