The lemon tetra is a deep-bodied South American characin whose soft yellow flanks, red upper eye, and black-edged fins only appear when the fish feels secure - which is a tank size question in disguise. Adults reach about two inches, and the species is a dedicated schooler, so plan around a group of eight to ten. That group needs a 20-gallon tank at minimum to school, turn, and settle properly; in smaller quarters or smaller numbers, lemon tetras stay skittish, pale, and pressed against the plants. It is one of the tetras that tells you plainly whether its home is right.
Numbers first, then gallons
A lemon tetra alone is a nervous fish, and a group of four is barely an improvement - the school's confidence seems to switch on somewhere around eight fish. At that size the group holds in open water, feeds without darting, and turns together with a flash of silver and lemon that makes the species worth its tank space. The deep body needs turning room too - a cube makes the school bunch rather than glide. Ten to twelve in a 20- to 30-gallon is better still. Buy them together from the same shop tank where possible; a group that arrives together settles together, while single additions spend days being inspected by residents before they are allowed to belong.
Water and light that flatter the fish
Lemon tetras come from slow tributaries in the Amazon basin, where the water runs soft and slightly acidic under forest shade. They adapt to ordinary tap water conditions in most homes, but their color is honest feedback: rich lemon and a bright red eye in soft, slightly acidic water under softened light, washed-out grey-yellow in hard water under glare. A dark substrate, background planting, and some floating cover pull the color forward. Keep temperatures in the mid-70s Fahrenheit and the water clean with regular small changes; this species notices neglected water in its fins and appetite before many tank mates do.
Tank mates in the same mood
Choose companions that share the lemon tetra's calm: other mid-sized tetras, corydoras on the bottom, hatchetfish at the surface, and peaceful dwarf cichlids all make good neighbors. Avoid fin-nippers - a lemon's tall anal fin invites attention - and avoid anything big enough to hunt a two-inch fish at night. In a mixed tetra community, keep each species' school at full size rather than collecting pairs of everything; three proper schools look and behave better than eight remnants. Feed a varied small-food diet and the males will show their best lemon when displaying to females at lights-on.
A shared record for a shared tank
When several people keep one tank, the stocking plan lives in someone's memory and drifts - a pair added here, a replacement there - until the schools are the wrong sizes. Write the plan down once and keep it where the whole household sees it. KinsLe's Family Sharing keeps this tank's profile, including the lemon tetra school's target size and tank mates, in front of everyone, so additions get checked against the plan instead of someone's recollection of it.