that used to happen to me after i fed. it could be wen u put the food in the tank, the flake has air in it. when the fish goes up to eat it, he is consuming the air in the food. this will bloat it until it well.... releases it. to avoid this soak the food in a cup of tank water, then dump the cup in the tank. the food will have absorbed some water, and when ur fish eat the food, it will be consuming less or no air.