Tetras are a group of mainly quite small, schooling fishes which originate from Southern and central America and Africa. Most grow to only 5 or 6 cm, with some reaching only 3 or 4 cm in length. However, a few types grow to around 10cm or more. Around one thousand species have been described from the Americas and two hundred from Africa, although many more are probably undescribed.
Tetras are distinguished by the presence of an adipose fin, that is, a small fleshy fin on the back behind the dorsal fin. This is present in most, but not all species, and serves no known function. Some unrelated fishes, such as corydoras catfish, also have an adipose fin.
In general tetras occur in clear well-oxygenated water, often amongst plant cover. The waters they come from are usually soft to moderately hard, slightly acid, and often tannin stained. Most species will adapt to a range of water conditions so long as the water is clean and well aerated. Extremely high pH (e.g. over 7.6, or 8 for hardy species) and very hard water should be avoided.
Some species that are wild caught are more demanding of water quality and must be provided with very soft and slightly acidic water in order to thrive, but these are not types which are commonly seen offered for sale.
Providing plenty of plant cover (either live or artificial) and using a blackwater extract will make most tetras feel at home and they will display their colors accordingly. Most tetras will fade in color if they are stressed or uncomfortable with their surroundings. (But note that many lose all their color at night, which is not a cause for alarm).
The vast majority of tetras are peaceful fish that mix well with other species, however, a few types are prone to being nippy (e.g. Serpae tetras). The size of tank mates should be taken into consideration, especially when keeping the smaller species such as neons and glowlightes. Large fish such as Gouramis, Angelfish and Silverdollars should be avoided: even quite passive fish will eat other fish if they can fit them in their mouths! Of course, larger tetra species such as buenos aires tetras, congo tetras, serpaes and others can be mixed with quite large tank mates.
Nearly all tetras are suitable for planted tanks and do not bother live plants at all. However, a few will nibble on very fine leaves, and the buenos aires tetra is a noted vegetarian.
In general tetras swim in the middle levels of the aquarium, but will come to the top to feed.