A good rule-of-thumb=fishes stomach are about the size of their eye-adult fish need 1-2 times a day feeding and babies 3-4 times per day feeding-
and yes the fish always seem to act like they are starving-to-death-mine beg for food every time I get close to the tanks -but most times the fish are not really so-much hungry as conditiond, meaning you=food

Over feeding can cause water problems and constipation problems, its also a good idea to feed a varity of foods too, I have special treats I give my fish like very small amount of fresh seafood and/or pieces of fresh water fish, bananas, cumcumbers slices depending on the type of fish-they like it and it makes me feel like I pleased them...LOL