A tetra that outgrows the label
Congo tetras sit oddly in the tetra family: where most tetras stay small and neat, a male Congo grows to around three inches, trailing long, flowing fins and a body that shifts through iridescent blues, golds and violet as it crosses the light. In their home waters in the Congo River basin they live in schools in open water, and everything about the fish - its size, its cruising gait, its display fins - assumes company and space. Bought as a pair for a small tank, a Congo is one of the hobby's common disappointments: grey, jumpy and stunted-looking. Bought as a proper school for a long tank, it is one of the great community fish.
Six or more, with both sexes
Six is the minimum group, and the school works best with a mix of males and females. The males' colour and finnage are display equipment, developed for an audience: kept with rivals to posture at and females to court, males colour up and extend those flowing fins; kept alone or as a lone pair, they often stay comparatively drab, as if the finery were not worth the effort. Numbers also buy confidence. Congos are not aggressive, but they are sizeable, fast fish, and in a small group they startle easily, bolting at shadows and movement outside the glass. In a school of six or more, a startle passes through the group and settles in seconds instead of turning into a panic.
The real requirement is length
More than the headcount, it is the tank's footprint that decides whether a Congo school works. These are open-water cruisers that swim in loose formation, so they need a long tank - the kind of length a thirty-gallon long or larger provides - with an open mid-water highway and plants massed at the back and ends rather than scattered through their lane. Height matters far less than length, and a tall, narrow tank is the wrong shape however many gallons it holds. Fit a secure lid: a startled Congo is a powerful jumper. If your current tank is a small community cube, the honest answer is that it cannot house this school, and a smaller tetra species will give you more pleasure in it.
Living with the school
A settled Congo school is peaceful but bumptious at feeding time, arriving at the surface in a rush that can outcompete tiny, slow tankmates - so pair them with fish of a reasonable size and feed in more than one spot. Their colour is not instant: young Congos are plain, and the males' iridescence and finnage build over months on a varied diet and clean water, which makes a growing school quietly rewarding to watch. Chasing within the school is display, not fighting, but a fish that is constantly excluded, pale and thin is one to watch. With the tape measure out and the Congo school still in doubt, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.