Why is my Oscar growing so slow?

I got him at the end of Feb, and he was 2", from head to tail. Now, in mid-June, he's only 4 inches. Here's his diet: beefheart 1 time a week, bloodworms 1 time a week, and some new life spectrum every day. His colors are awesome! But he is growing so slow. Any ideas as to why? I keep his water in pristine condition, and the plants (he hasn't torn up :) provide some good filtration, so the water is good! And his belly always bulges slightly after I feed him, so I'm not skimping out on the food. Hmmm, help!