Get the biggest sump you can fit under the tank! The bigger, the better. By adding more water to the system, it becomes more stable. I recommend getting a sump custom built by an acrylic fabricator. Look in the yellow pages under "Plastic fabricators." Measure the space under the tank, and get one that fits in there. Leave at least 1/2 inch of space so you don't get stuck with something that is too big! Look at some sumps in fish stores, catalogs, and online to get ideas of how to make it. You can order bulkhead fittings through the mail, or your LFS may stock them. I don't like to use submersible pumps, as these tend to add more heat to the tank than an external pump. If you use a glass tank for a sump, you will be forced to use a submersible, or drill the tank, which is difficult. Drilling acrylic is a breeze. Be sure to put a valve on the pump inlet side in case you need to change the pump.
I recommend using CPR brand overflows. Decide which pump you'll be using, and get one that has a flow rate similar to that pump. Be sure you know what the pump is rated for at the height you will be pumping to.
Good luck,
Sonny
