What will he do once they're full grown, freakishly fast or not? It's best to start off with a tank that doesn't have to be upgraded to begin with. Just my two cents.
I couldn't disagree more. I like starting small and working up with the fish as they grow. It makes it easier to treat and makes the tank more interesting.
I would also like to see a 12" red belly piranha and meet the person who has had it for that long.