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When and How Much To Feed
Out in the wild, food is not plentiful year round. In comparison, your tank is a feeding trough. Chances are, fish will get much more food in your tank than in the wild. Thus, the problem to avoid is overfeeding, rather than underfeeding. Fish are also smart (in the sense of Pavlov’s Dog) that they learn quickly how to train their masters into giving them food. Just because they act starved when you walk by the tank an hour after their last feeding, doesn’t mean they actually are. Look at the abdomens on your fish. If they are fat and bulging, your fish isn’t starving. If they are thin, or hollowed out (concave upward), you may want to consider increasing the feeding.
Fish can be safely fed twice a day, once a day (or less). If your goal is to maximize growth, feed them more frequently, but smaller portions.
There is an old rule that says fish should be fed no more than they eat in 5 minutes. Like most general rules, this one is only partly correct. What you really want to avoid is having food to rot in your tank uneaten, which negatively impacts water quality. However, some fish eat food slowly and will take 30 minutes to eat even a small portion.
How long can fish go without food? Healthy fish can easily go a week without food. When you go out of town for the weekend, don’t bother getting someone to feed your fish. (Indeed, someone not familiar with fish tanks is likely to overfeed your fish while you are good, leaving you a mess to deal with when you return.) In addition, stay away from those “vacation feeders” that slowly dissolve. They can upset the pH of your tank and lead to excessive food in your tank.
Source: users.cs.duke.edu/~narten/faq/chemistry.html