The short answer for cardinal tetras
Feed cardinal tetras once or twice a day, and keep each meal genuinely tiny — what the school clears in one to two minutes, with nothing left drifting. For a typical school of ten, that is a small pinch of micro food, far less than most new keepers expect. Two modest meals suit growing or breeding-condition fish; one careful meal a day maintains healthy adults in a mature tank, provided the amount is right rather than simply frequent.
Small mouths set the menu size
A cardinal's whole body is only a few centimetres long, and its mouth is proportionally minute. Standard flake pieces and regular pellets are simply too big — the fish mouth them, spit fragments, and half the food becomes waste. Choose micro-pellets, finely crushed flake rubbed between your fingers, or frozen baby brine shrimp and daphnia, which cardinals take eagerly and which help keep that red stripe vivid. Offer food in the middle layer of the water column where the school hovers, letting pieces sink slowly past them rather than blowing them across the surface with a strong current.
Why clean water matters more with cardinals
Cardinals come from soft, acidic blackwater and are less forgiving of pollution than hardier tetras. Uneaten food in warm water breaks down quickly, and a school fed generously twice a day can foul a small tank faster than the filter keeps up. Feed sparingly, siphon the substrate lightly during water changes, and let the fish themselves tell you the portion is right: after a meal their bellies should round slightly, and within the hour they should look normal again. Persistent plumpness, cloudy water, or algae creeping up despite good light control all point to the pinch being bigger than the school needs.
Feeding a shy school evenly
Newly settled cardinals may hang back in plant cover while bolder tankmates feed. Scatter the micro food in a loose line along the front of the tank so shy fish can dart out, grab a piece, and slip back, and consider dimming bright light for the first minutes if the school seems reluctant. Avoid housing cardinals with large or boisterous feeders that monopolise every meal; if they share with other small species, watch a full feeding to confirm the cardinals — not just the tank — are eating.
Warning signs at feeding time
A cardinal that stops schooling, loses the brightness of its blue and red bands, or refuses food while the rest feed deserves attention the same day. Check temperature and test the water first, since cardinals react quickly to ammonia or a sudden shift in acidity. One fish wasting away while the school thrives, or several fish going off food together, is a reason to seek advice from an aquatic veterinarian or experienced fish retailer promptly rather than increasing feedings to compensate.
One pinch, one person, no guesswork
With portions this small, the real danger is two household members each adding 'just a pinch' an hour apart. Agree who feeds the cardinal tank each day and make the completed feed visible to everyone else. KinsLe's Care Teams does that for this school: the daily feeding is assigned to a named helper, and once it is marked complete with its timestamp, nobody else adds a well-meant second pinch.