You could do a blue Cray for the bottom dweller I'm sure your son would love it then some sort of a mid to high swimmer that has the same water paramaters as the Cray being a guy I would have loved to have a Cray as a pet when I was younger.
It's funny you mentioned the cray. My son and I were just discussing that tonight actually. I asked if he'd like a blue one and he asked if they came in pink. *hides face* lol! I'm a little concerned about the dead bugs all over this thing. I've had some pretty dirty tanks before, hard water, algae (red,brown,green,black) but this is a first for me to have one with bugs?