red tails seem to grow no matter how much or little you feed them, and they eat ANYTHING that fits in their mouth. that includes tankmates.
don't know what size tank you've got him in, or what his tankmates are.. just be warned.. they grow amazingly fast when they are young but slow down around 12 inches.. and they apparently get to about 5 ft in the wild, but not quite that big in captivity. so you're going to need a big tank sooner rather than later.