Peaceful, at a larger scale
A Congo tetra's manners are pure tetra — school, drift, display, ignore the neighbours — executed by a fish two or three times the size of the tetras most keepers know. Males colour up with an opalescent blue-green sheen and extended, veil-like fins, and their displaying is aimed entirely at each other and the females. Nothing in the species is aggressive; the community question is purely physical. What fits safely alongside a cardinal tetra will not necessarily fit alongside a fish that can swallow small tankmates whole while simply feeding enthusiastically in mid-water.
Tankmates sized to match
Build a Congo community from fish of comparable presence: larger rainbows, medium barbs of peaceful species, bigger rasboras, cichlids that stay low and calm, and catfish that keep to the bottom. Most adult community staples are fine. What to leave out is the miniature end of the hobby — ember tetras, chilli rasboras, baby shrimp, fry — not because Congos hunt them deliberately but because a feeding Congo is a large mouth moving fast through the same water. Shy, slow feeders of any size also fare badly, since a Congo school reaches scattered food quickly and thoroughly.
Space and group requirements
Congos use the whole length of a tank and look cramped and washed out in short aquariums; a long footprint, open mid-water and planting along the back and ends suits their travelling school. Six is a fair minimum and eight or more settles the males' displaying into the group, where it belongs. They are not difficult about water by blackwater-tetra standards, but they colour and breed best in clean, slightly soft, well-maintained conditions, and a group kept in a neglected tank fades to a grey shadow of the fish in the shop — usually the same fish, weeks later, in better water there.
Feeding and everyday care
Congos are hearty omnivores that take flake, pellets and frozen foods with enthusiasm, and benefit from some vegetable matter in the mix; feed for the school's size, because these are substantial fish whose waste load matches. Watch new arrivals for the first fortnight — they can be shy travellers that hang back while residents feed, and a quiet extra feed at their end of the tank bridges the gap until they join the school's confidence. A settled Congo group is long-lived and spectacular.
Congos in the long-term community
A Congo community is a commitment to a particular scale of fishkeeping, and it rewards planning in years rather than months. The group will outgrow a marginal tank while still young, so start them in the aquarium they will finish in, with companions chosen for adult size on all sides — including the bottom, where a growing school appreciates robust catfish company rather than delicate species they blunder into at feeding time. Keepers who make that investment get a centrepiece with genuine presence: large, shimmering, long-lived fish whose displays and group travel give a big tank a wild, riverine feel that small tetras, however lovely, cannot produce at that scale.
Should the group move on, KinsLe Transferrable Profiles keep its size, diet and tank requirements recorded for the next keeper.