I will be ordering my overflow and return kit from glass-holes.com very soon. They offer quite a few different overflows and I am unsure of which one to get. I think I would need the 700 GPH for a 40 gallon breeder, correct? As far as the return, I would go with the 3/4 inch return. Correct me if I'm wrong. I read somewhere that 1" pvc is used on the overflow and 3/4" is used on the return. Is that true? I am not sure how often the water in the tank should be turned over, so I would appreciate it if someone could tell me. Not sure if this makes a difference, but I will be running 2 Hydor 425s in the tank.