After three weeks of having my fish, one of the black tetras is feeding off the bottom. I feed them flakes and he will go for them at the top. He mostly chews up the flakes and spits out the smaller pieces. I catch him a lot bringing his mouth down towards the gravel and did an experiment to see that he is feeding off the bottom. Should I continue to feed him just the flakes, and let him feed off the bottom, or should I maybe feed him something different? Also, how often should I feed my fish bloodworms?