A fish begs the moment you walk past the tank, and it looks hungry even when it is not. That is why vets and welfare groups say the same thing: the most common feeding mistake is giving too much, not too little. Uneaten food sinks, rots and breaks down into waste that pollutes the water, and poor water quality is the most common cause of illness in aquarium fish.
The fix is small meals, the right rhythm and a bit of variety. Here is how much that means in practice.
The RSPCA's fish advice is to feed once a day, and only what the fish can eat in under two minutes. An exotic-pet vet gives a way to start: offer 4–5 flakes or pellets per fish, add a little more only if they are eaten right away, and stop as soon as food starts to sink. If food reaches the bottom, that was too much.
Sources do not agree on every detail. Some keepers say two minutes, others three, and the pieces-per-fish number is a starting point rather than a law, because every tank, species and temperature is a bit different. Watch your fish and adjust. Fish that live in a group should be fed in two or more spots so the bold ones do not take everything.
Start with a pinch, watch it vanish, and stop when food starts to sink.
A brand-new aquarium has not yet grown the helpful bacteria that clean up fish waste (see our guide to the nitrogen cycle). The same vet suggests feeding a new tank only every other day for the first month, then moving to once a day. Small feeds mean less waste for a young filter to deal with.
Variety matters too. A good flake or pellet food is the staple, with other foods on other days: the vet suggests the staple three or four times a week, and frozen foods at least twice a week. Fish that feed near the bottom need food that sinks, not flakes that float. Use the planner to see what a week could look like for your tank.
If your fish stop eating, look swollen, hang at the surface or clamp their fins, that is not something to fix by changing the food. Ask a vet who sees fish, and check the water first.
Pick a food made for your kind of fish, such as a quality flake or small granule, and add occasional frozen foods like brine shrimp or daphnia. Skip meat and offal from mammals, because fish digest those fats poorly. Buy dried food in small tubs, keep it cool and dry, and replace it when it is old. One vet source says use it within about two months of opening, the RSPCA says discard anything open for over a year, so smaller tubs and fresher food is the safe side of both.