Overfeeding aquarium fish is understandable, because feeding your fish is the primary way in which you interact with them – and besides, they are always so pleased to see you coming with a can of fish food.
However, more aquarium fish probably die from overfeeding than from underfeeding, and I am pretty sure that even if they were fed very little, very few aquarium fish have ever starved to death. Fish in our fish aquariums get more food, with less work, than would ever be the case in the wild. In addition, your aquarium fish don’t have to worry about becoming a meal for some other creature.
Healthy aquarium fish will always look hungry. In fact, a good way to test the health of the aquarium fish at your local fish store is to simply put your hand over the aquarium. In a good store, where they feed their fish regularly, the fish should come rushing up to the surface in anticipation of food. (It should be noted, however, that it takes a few days for the fish to get the idea, and there will always be aquariums of new fish in any local fish store that won’t immediately respond in this manner.)
The cardinal rule when it comes to feeding your aquarium fish is to make sure they eat everything you feed them. Usually, this is put in terms of “all they can eat in ‘X’ minutes,” where “X” is a number between three and five. My opinion is that “X” should be .5 – yes, that’s right – half a minute. If your aquarium fish have really cleaned up all the food in half a minute, then it’s all right to give them the same amount of food again. Feeding your aquarium fish like that once a day is fine. If you want to feed them twice a day, or even more frequently, that’s fine, too — as long as they eat it all in half a minute.
Even without lots of aquatic plants, aquarium fish are fine without fish food for a week. In fact, one of the best things you can do for your aquarium fish is to not feed them one day a week. My aquarium fish often will go unfed two or even three days in a week.
All of the prepared fish foods that are available in your local fish store are high in quality and provide good nutrition for your aquarium fish. You should supplement the diet of dry prepared fish foods with some frozen or freeze-dried fish foods three or four times a week. It’s also a good idea to use a number of different dry prepared fish foods. For example, the aquarium fish in my 300-gallon display aquarium get fed three different dry foods – spirulina algae wafers, high-protein shrimp pellets, and frozen mysis shrimp and bloodworms. The aquarium fish are fed once a day, five days a week.
The bottom line is to feed your aquarium fish a varied diet, feed them only as much as they eat in half a minute, and don’t feed them at all one day a week.
The reason that overfeeding is a very bad thing for your aquarium fish is that it pollutes the aquarium if there is uneaten fish food left to decay on the bottom of the aquarium. Uneaten fish food just adds to the ammonia levels of the aquarium, and it can very easily result in more bacteria than the nitrogen cycle can handle. It’s like when the bowl of fruit you have sitting out on the kitchen table goes uneaten.