It all really depends on what kind of fish and size of tank. For example if it is a small tank where you have clown you can get away with feeding them once a day in order to keep nutrients low. Similarly, less active fish like gobies, blennies, etc dont need to be feed that often. On the other hand with a tank full of bigger fish with multiple tangs, anthias, butterflies, angels, etc they need to be fed multiple times a day with smaller feedings in order to keep aggression down and keep them healthy.