Many people prefer to feed fish twice a day, once in the morning and again in the evening. However, in an attempt to mimic natural feeding habits, you should feed very small amounts of food four to six times a day if possible, being careful not to overfeed. In the absence of natural light, make sure a lamp is on so your fish can see the food. If caring for fish is a family affair, keep track of who fed which fish last, and let other members of your household know so your fish will not be overfed.
Fish will overeat, and excess food will not only cause your fish to grow fat and unhealthy, but excess food will contaminate your water. Most fish that you buy for your tank are young, and how much they eat can affect their growth. Overfed fish can grow up with deformed bodies.
As a rule of thumb, don't feed your fish more than they can eat in about three minutes. To determine how much that is, drop a small pinch of food into the tank and see how long it takes the fish to eat it. Observe how much food falls to the bottom, and how long it takes the bottom-dwelling fish to eat it. If after two minutes there's not a scrap of food left, put in another pinch. So long as you carefully monitor your quantity, minute amounts of uneaten food should not pose a problem. But excess or visible food should be siphoned out of the tank using a specially designed water-siphon vacuum. This should be done after about 15 minutes, before the excess food can contaminate and cloud your water.
As you watch your fish eat, you'll probably notice that some aren't getting their share of food. You can help by varying the kinds of food offered. For instance, if the fish that usually swim at the top of the tank need more food, offer flakes because they float longer. If bottom-feeding fish don't seem to be getting enough food, give them heavy foods such as pellets that sink to the bottom of the tank. Another way to help food-deprived bottom feeders is to briefly turn off both the tank lights and room lights after you drop in the food, so the other fish won't eat it before it reaches the bottom of the tank.