He is a cute and adorable fish.
Oscars I find are quite temperamental. Like spoiled children. He may not like a plant you have in the tank for all you know.
He may be bored with the food you are feeding him.
He may be mad because you don't spend enough time fawning over him.
If you trust your testing routine, and you are sure there is no chlorine in the tank, and you are not over feeding him, it should be fine.
When I first started out, I killed two baby oscars by overfeeding them
Finally, when he does hit 12" you should still call him pipsqueak. I like oxymoronic names. My mom has a teacup chihuahua I suggested she named Brutus. Tough name for a dog who weighs 5 pounds on a heavy day!
Noddy, I guess I am an official oscar keeper, since I started my own HITH thread last week. YAY