Why numbers matter more for cardinals
In the wild, cardinal tetras move through flooded forest and slow blackwater in shoals that can number in the hundreds, drifting between dappled light and cover. A cardinal's famous colour - the electric blue line over a body red from nose to tail - is partly a signal to its own shoal, and it is honest: a cardinal that feels secure shows it fully, while a frightened one literally fades. That makes the school size a welfare question for this species, not just an aesthetic one. Too few cardinals and you will not see nervous behaviour so much as an absence of behaviour: fish hovering separately in the plants, colours washed out, feeding half-heartedly.
Six is a floor, ten is a school
Six cardinals is the accepted minimum, the smallest group in which the fish will school at all. Ten or more is where the species becomes itself: the group cruises the open middle of the tank together, turns as one, and holds its colour through the day. If your tank and filter can support it, a school in the low teens suits cardinals better than the minimum ever will, and it is the single most effective upgrade you can give them - better than any change of food or lighting. Plan the tank around the school you want rather than buying six now and meaning to add more someday; cardinals kept waiting at minimum numbers are spending those months stressed.
The condition attached: a mature tank
Cardinals carry a reputation for delicacy that really belongs to the tanks they are put into. They are sensitive to the ammonia and nitrite swings of a newly set-up aquarium and to sudden changes in water chemistry, so the school should go into a mature, fully cycled tank whose readings have been steady for a while. Acclimate them slowly - floating the bag and adding small amounts of tank water over time is kinder than a quick tip-in - and keep their water clean and, above all, stable. They come from soft, acidic water, but a settled group adapts to reasonable local conditions provided nothing swings abruptly. Acclimation and stability matter more to a cardinal than chasing an exact textbook figure.
Keeping the school whole
A school shrinks quietly over time - an old fish here, an unexplained loss there - and cardinals notice the thinning before you do, drifting back to plant-hugging as the number falls. Replace losses to bring the group back up, adding a few at a time to the established tank and watching the newcomers settle. Keep an eye on the stripe as your daily health check: a cardinal that has lost its colour, separates from the school, or stops feeding is showing you the earliest version of a problem. Test the water first; if readings are clean and the fish does not recover within a day or two, take advice from your vet or an experienced aquatic shop. Family Sharing on KinsLe, meanwhile, extends profile access for the cardinal school to relatives the owner has picked.