In nature, fish search for food when are hungry, but will also eat whenever food is available.  If food sources are plentiful, they will eat several times a day. On the other hand, if food sources are scarce, fish might go for days between meals with no ill effects. For this reason, fish are very opportunistic and will eat whenever they have a chance. 

How often should be feed

The correct frequency of feeding will vary based on the species of fish. In general, most fish do quite well on one or two feedings per day. Most fish require 16 to 24 hours to fully digest the food they eat, so a once-a-day feeding is quite sufficient. However, some owners prefer to feed their fish very lightly twice a day. Whether you feed one or two times per day, the key is to keep the feedings very small. The timing is not critical, with the exception species that are nocturnal feeders, such as certain catfish. If you have nocturnal fish in your aquarium, be sure to feed them just before turning the lights out at night.

There are some exceptions to the once-per-day feeding rule. Herbivores (vegetarians) need to eat frequently because they do not have large stomachs to hold a lot of food. In nature, they graze all day long on plants that are readily available and have no need to gorge opportunistically. These species should be given several small feedings a day or provided with live plants they can nibble on constantly.

Also, newly hatched fry and young fish not fully grown require more frequent feedings of special foods designed for fry. usually, three or four feedings a day are necessary for fry.

Determine the amount of feed

As for how much food to feed, a good rule of thumb is to feed no more than the fish will consume completely in less than five minutes. When in doubt, underfeed! You can always give them another small feeding if necessary. However, if you overfeed, the uneaten food will produce wastes that can be harmful to the fish.

As a final note, keep in mind that not only the amount but the type of food is important. Research fish species to determine which type of food is appropriate for the fish you keep.

 Some fish foods are specific to species, such as algae wafers for bottom feeding herbivores, while others may be tailored to freshwater fish or saltwater species. Whatever food you choose, make sure it is balanced to provide the vitamins, protein, and minerals your fish need. Small amounts of good nutritional food are much better for your fish than large amounts of cheap food.