there are 2 types of overflows. ones the are built into the tank. this is what you want. then there is what I have.... and external overflow. it's basicaly two boxes with a siphon tube. one box goes in the tank and the water overflows into this box. the other hangs of the back/side of the tank and the water siphon the box in the tank the the box hanging of the tank and then down a hose to your sump! the problem with this one is if the siphon breaks you have a mess!!!!