Twenty gallons, long, for a school of eight or more
The cardinal tetra's red runs the full length of its body, and the way to see it properly is a school moving as one ribbon — which sets the tank size. Twenty gallons is the working minimum, in the long format, housing eight to ten cardinals with room to school along the front glass. A larger tank, thirty gallons and up, is where cardinals become spectacular: schools of fifteen or more hold together over distance, and the extra water volume also buys the stability this species demands. Ten gallons is often quoted for a few cardinals, but a few cardinals is the wrong unit. Kept in small numbers in small water they are shy, pale and short-lived; the tank that suits the school is the tank that suits the fish.
Stability is the second half of the answer
Cardinals come from the blackwater tributaries of the Rio Negro — warm, soft, acidic and above all unchanging. They tolerate a reasonable range in the home tank, but they punish swings: a big cold water change, a neglected filter, a new tank still cycling. That is why the setup should run for weeks with plants and filtration established before the school arrives, and why water changes should be smaller and more frequent rather than heroic and rare. Their sensitivity is relative, not fragility — a mature twenty-gallon with steady habits keeps cardinals for years — but it separates them from the hardier neon tetra, and it is the reason tank maturity appears in any honest answer about housing them.
Layout and companions for cardinals
Dim the light with floating plants, darken the background, and plant the back and sides so the school owns a shaded front lane. Warm water in the upper seventies to low eighties and soft, slightly acidic chemistry complete the blackwater picture; driftwood both releases gentle tannins and gives the scape the flooded-forest feel they recognise. Companions must be peaceful and similarly sized — other small tetras, pencilfish, dwarf cichlids of calm species, corydoras and otocinclus below. Anything big enough to hunt a one-and-a-half-inch fish cancels the calm the tank size was bought to create. Feed small live and frozen foods regularly; cardinals colour noticeably better on them than on flake alone. Finally, judge a cardinal school by its mornings. In a tank that suits them, the group leaves its night positions together as the light ramps up, colours building over the first half hour into the full red-and-blue ribbon. A school that stays scattered and dull well into the day is telling you something about water, numbers or peace in the tank, and that daily read is only available to keepers whose group is large enough to behave as a group — one more thing the twenty-gallon-long minimum quietly buys.
When the school needs decisions
Cardinals reward a keeper who notices drift early — colour fading across the group, schooling that loosens, fish separating at the surface. Those signs call for a water test and a look at the school count before anything else, because both problems are fixable when caught small. Some questions sit between a water test and a vet visit — a planned tank mate, a fading school. For those, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.