Ever notice when you walk over to the tank that your fish swim over like they are starving and just can't get enough food. Well as tempting as it maybe you really want to limit the amount and amount of times you feed them.
Fish should only be fed once or twice a day and the amount shouldn't be more than they can eat in 1 to 2 minutes. It's always easier to add a little more than to take it out. As long as your feeding the fish a high quality fish food you won't have to worry about killing your fish by feeding them.
With the small fish like livebearers you should feed the twice a day. With the cichlids and larger fish you can stick to once a day unless you have overly aggressive tankmates, then you can feed them twice to help deter aggressive behavior.
I usually measure the amount of food to feed them with a small pinch between your thumb and pointer finger for 5-6 small fish. If your afraid of over feeding look for fish foods that don't cause cloudy water or have a lesser chance of polluting the tank. These aren't sure fire ways to stop overfeeding but it does help. Cichlids and other large fish do best with pellets because they are easier to measure the feedings and don't break apart as easy. Flake food tends to fall apart and pollute the water. Also if your afraid you overfeed your tank take a look at How to reduce nitrates in aquarium. Nitrates can raise because of uneaten food and can stress your fish out. Now that you know how to feed your fish you should look at breeding them.