Young Macdonald's Farm Fish Feeding Once the size of the fish is determined by weighing the fish on a scale the primary investigator can calculate the amount of feed per day. The basic rule is that small fish are generally fed at a higher rate then larger fish, so as the fish grow the percentage of body weight being fed reduces. For Example: 20 gram fish 6% 50 gram fish 5% 100 gram fish 4% 200 gram fish 3% 300 gram fish 2% 400 gram fish 1.5% The weight of the fish should be checked at least every two weeks! Feeding should be broken up to approximately 3 feeds per day.