Almost anything. Mine along with other inhabitants get flake regularly, frozen bloodworm etc every so often and live bloodworm/brineshrimp etc as a treat now and again. The frozen stuff i usualy buy a quatra pack that ahs bloodworm, daphnia etc in
My tetras like flake, bloodworm, daphnia, brine shrimp etc.
feed the live stuff as treats every week if you wish. There is lots of brands of flake, or pelet foods your tetras will take, just buy small packs and see which one they like the most, and make them look the best