Can Cardinal tetra Live With Congo tetra?

The congo tetra is peaceful, but it is an 8 cm fish that swims hard, and its own profile warns that it can frighten shy species with that activity and its size. A 3.5 cm cardinal is the shy species in question. This is not about predation; it is about a small fish spending its day keeping out of the way.

The water agrees: both take 23 to 28 °C and pH 6 to 7.5, and both are shown off by a dark substrate and planted cover.

Tank size decides it. Congos need a mixed sex shoal of six or more and a tank at least 120 cm long, about 120 litres, for their swimming, and cardinals want 10 or more of their own. At that size, with planting at the sides and open water in the middle, the two settle into separate parts of the tank. In anything smaller, leave the congos out. One more point in their favour as company: congos must be kept away from fin nippers, and cardinals are not that.