The group is what makes the colour
A single lemon tetra is a pale, translucent fish that fades into the glass. Six or more together, settled and confident, develop the lemon-yellow body wash, black-and-yellow dorsal fin and red eye that give the species its name. Plan on six as the minimum and eight to ten as the display range, housed in a tank of twenty gallons or more with length for schooling runs. Their deep, disc-shaped bodies need a little more water per fish than slim tetras of the same length, so count them honestly in a stocking plan.
In the Rio Tapajos tributaries they come from, lemon tetras hold in large shoals over sandy stretches between plants. The home tank that copies that picture — open centre, planted margins, subdued light — gets the behaviour and the colour together.
Schooling behaviour to expect
Lemons are midwater fish with a lively but peaceable style. Males in a good-sized group spend the morning hours displaying to one another, fins fully spread, in contests that never turn damaging. Females cruise slightly deeper-bodied among them. This daily theatre disappears in small groups, where the fish instead hover nervously near cover — one of the clearest examples in the hobby of numbers creating the pet, not just populating the tank.
They are also jumpers when startled, so a full lid matters, especially in the first week while the group is still mapping the tank.
Tank mates and water for lemons
Warm, soft and slightly acidic water matches their home rivers, and stability matters more than hitting a precise figure. Good companions include other mid-sized peaceful tetras, small corydoras, dwarf cichlids that mind their own business, and hatchetfish for the surface. Avoid fin-nippers and much larger fish that could treat a two-inch tetra as food once the lights go out.
Feed a varied diet with some colour-supporting foods — small live or frozen items a few times a week make a visible difference to the yellow within a month of steady group living.
Breeding behaviour sometimes surprises lemon tetra keepers, and a full group is the trigger. Males in condition scatter eggs among fine-leaved plants at first light, and while almost none survive in a community tank, the dawn activity explains sudden bursts of colour and chasing that alarm owners who have only ever seen a small, subdued group. It is a compliment to your stocking, not a problem: the fish only attempt it when numbers, food and water all tell them the shoal is secure.
Watching numbers and replacing losses
Because lemons are bought for their collective effect, a school that slips from nine to five quietly loses the point of keeping them. Count during feeding, when the whole group rises together and missing fish are obvious. Top up with at least three at a time, quarantined where possible, and add them after a water change so everyone meets in freshly settled water. Logging each change of count in the household's shared tank notes keeps the picture current between shop trips. With the profile shared through Family Sharing in KinsLe, whoever feeds or shops next is looking at the same lemon tetra school everyone else sees.