This short article is based on the answer I gave to a friend on an aquarium discussion list, who asked the exact question above. Since I’m not a biologist, some of the things I wrote below are no more than educated guesses, but there were several biologists on that list and nobody seemed outraged by what I wrote, so I guess it can’t be that bad

As with almost everything related to fishkeeping, there’s simply no definitive rule on feeding, because it depends on too many factors. But there are several general guides that can be used based on collective experience. The most common one, which has always worked for me, is to feed 2-3 times a day, as much as your fish can eat in less than 3 minutes. Of course there’s a lot of tolerance here, but in general if after 5 minutes there’s still food in the tank, you should reduce the amount you’re feeding. This works well for beginner, community and other kinds of tanks with a reasonable mix of species. Single species, breeding and hospital tanks will require a more careful approach, based on the specific requirements of their inhabitants.

There are some important things to remember about fish feeding habits. In nature, most of them usually have a hard time finding food, and through evolution they have been “programmed” to eat as much as they can fit into their bellies whenever they find some, because they never know when the next meal will be found. In a tank, we do know when their next meal will come. Therefore, it’s up to us to control the amount they get. Feeding them until they seem satisfied is not a good idea. Fish are expected to spend most of the day feeling hungry and searching around for little bits of food. A common mistake made by beginners is to overfeed because their fishes seem to be desperately begging for food every time the owner approaches the tank, and he decides to satisfy their appetites every time. A hungry fish is a healthy fish…if they don’t come up to you when approach in the morning, that’s when you should start worrying.

As another evolutionary defense, fish seem to adapt to the availability of food, by adjusting their growth rate. When they live in a place with more food available they respond by growing fast, and vice versa. Because of this, I believe that the actual amount you decide to offer isn’t as important as maintaining the same criteria throughout their lives. So the ideal situation would be to setup your tank, populate it with young fishes, define a reasonable feeding criterion and stick to it.

Finally, keep in mind that it’s way easier to weaken or even kill your fish by overfeeding (because it spoils the tank water) than by making them starve to death. I know a story about a guy who owned a pair of Oscars alone in a tank. When one of them killed the other. He was so mad he decided to stop feeding the killer and let it die. After several weeks with nothing to eat the Oscar was still alive, and the guy gave up and took it to a fish shop. Fish can easily go for a several days unfed, and many people argue that making your fish fast once a week is actually healthy for them, it sort of cleans them up internally.