Depends on the fish...
"normal communiy" fish twice a day.
African Dwarf rogs once every three days.
Red clawed crabs once every other day.
Fry 5 times a day.
Senegal bichir once every three days.
Mondays are "fast" days...
The rest of the week they get 2 very small meals and a treat of either peas, dried krill, or a "grazing" meal of part of an algae wafer or sinking tablet.