Fish has their own growth spurt. Especially ones that arent wild. If you really feel as if they dont grow quick or they arent growing, then play with their food a little. Yes, Pellets from Hikari and NLS do have the additive nutrition that keeps fish healthy and should make them grow, but vary the diet. Im sure you are varying the diet, but what i mean is vary the nutrition. Like start feeding it like massive amounts of protein such as Beef Heart. If you arent satisfied, start putting in mealworms. What i like to do is get a bunch of meal worms, put it in a cage or container, and feed them hikari pellets of fish flakes. after they are full from eating start feeding the whole worm to the fish. That is a technique i like to use.