Over feeding will increase your nitrates and if you really over do it can cause ammonia spikes as the food breaks down. Also some fish will eat till they pop - literally. Also try not to feed at the same time eery day and vary the type. Fish can and will learn feeding time. My clowns recognize the flake food jar and go nuts when they see it. Even my watchman goby (mister anti-social) comes out when he sees the container.
Rememer that your tank is basically a closed system - whatever you put into the tank will eventually buid up. Water changes, nutrient removal via macro algae growth, skimming, and filters (live and mechanical) help to remove this build up, but you have to make sure you are not importing more than the systems can export.