hmm. perhaps it is not enough. my small lemon fins literally turn from elongated fish to round fish after a feeding - they bulge so much its like they are going to explode. they wolf down Tetra cichlid sticks (that are half their body length) like its nothing. they steal them from my redhooks, right out of their mouths LOL they are growing rapidly. an inch per month so far.
try either removing a whack of fish from that tank, or moving them to another tank, perhaps even by themselves and power feed.
maybe their growth rate will speed up.