Tetras are the backbone of the community tank, and the difference between a tank of tetras that looks incredible and one that looks washed out is almost always the same thing: how many you bought.
Six is not a school, it is a start
Most tetras are shoaling fish, and a small group is a stressed group. Stressed tetras hide, lose color, and nip — which is where the reputation of some species as fin-nippers comes from. It is usually a symptom of being kept in threes. Twelve of one species looks dramatically better than six each of two, and behaves better too. If you take one thing from this page, take that.
Match them to your water and your plants
Neons, cardinals, rummynose and most of the classic South American tetras come from soft, acidic blackwater and show their best color against dark substrate and heavy planting. They will live in San Diego's hard alkaline tap water perfectly well — captive-bred stock has been raised in it for generations — but they will look better in a planted tank with cover than in a bare bright one. Rummynose in particular are a live indicator: their red noses fade the moment water quality slips.
Which to avoid together
Serpae tetras and a few others genuinely do nip, and long-finned tank mates like bettas, angelfish and fancy guppies are the usual casualties. Buffalo-head-sized species like the Congo tetra want more room than a nano tank. And neons and cardinals look similar but neons are noticeably less tolerant of high temperatures — do not put them in a discus tank.
See also rasboras, corydoras and plants.
