yeah he should be ait in a 40. oscars double their size every 3 months when young, and start to slow their growth at about 8 inches. However, just cuz it is slowed down doesnt mean it isnt fast. They will still be growing about an inch every 1 to 2 months. I like them, but they eat and crap to much. Make sure you have a powerful filtration system on your 40. I had 2 in my 55 and it smelled so bad and I was vacuuming it twice a week. I had an emporer 400 on it.