The Congo tetra comes from the Congo River basin in central Africa, far from the South American waters most tetras call home. Males grow to about three inches, females a little smaller, and mature males develop elongated, white-edged fins and an iridescent sheen of blue-green over gold that shifts as they move. They are active, confident schoolers that use the whole length of a tank, and everything about stocking around them starts with that combination of size, speed and finnage.
A tetra big enough to set the rules
A three-inch schooling fish changes a tank's balance in ways a neon never could. Congos need at least six of their own kind to school properly, and the group needs length to swim in, so a long tank of thirty gallons is a floor and forty is better. Their mouths are sized for insect larvae and small crustaceans in the wild, which means genuinely small tank mates are not companions to them, they are food that has not been eaten yet. At the same time, those flowing male fins are exactly the sort of target that nippy species exist to torment, so Congos sit in a middle band: too big for nano fish, too fancy-finned for barbs.
Companions on the Congo's scale
Rainbowfish are a natural match, similar in size, equally active, and peaceful in the upper and middle water. Diamond and emperor tetras hold their own without friction, and larger rasboras school calmly alongside. Corydoras of any of the common species work the bottom undisturbed, while synodontis catfish, including the upside-down catfish, bring a fellow African river fish that shares the Congos' water preferences. A pair of kribensis can hold a cave territory at the bottom of a roomy tank without troubling the school above them.
Too small, too nippy, too aggressive
Neon and cardinal tetras are the painful classic: kept with adult Congos they disappear one by one, and the same applies to endler fry, baby shrimp and the smallest rasboras. At the other extreme, tiger barbs and serpae tetras will reduce a male Congo's fins to ribbons within days, because no amount of space distracts them from that trailing finnage. Aggressive or territorial cichlids beyond the mildest dwarfs bully Congos out of the open water they need, and a Congo school pressed into the plants loses the colour that made you want them.
Water and layout for a confident school
Keep Congos between 75 and 82°F with a pH from 6.0 to 7.5, on the soft and slightly acidic side if you can manage it. Build the tank around an open central lane for schooling, with planted margins and driftwood at the edges where the fish can retreat. Subdued lighting brings out the iridescence that bright white light washes flat, and a secure lid is essential because a startled Congo launches itself. Before committing to a larger tank mate, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.