And a example of how to build your own acrylic sump.
http://www.melevsreef.com/what_sump.html
Note:
The return pump is constantly trying to overflow the tank while the overflow is draining the tank, so assuming your overflow can handle equal to or greater than the capacity of the pump the tank will have equilibrium.
When sizing your sump do not size it just large enough to hold the additional water capacity. Make sure the sump can also hold any water that will drain back down the plumbing when you turn your pumps off or else you will have a flood.
Everything You Always Wanted to Know About Sumps, Part I
by Gregory S. Taylor
http://www.reefkeeping.com/issues/2003-01/gt/index.php
Everything You Always Wanted to Know About Sumps, Part II
by Gregory S. Taylor
http://www.reefkeeping.com/issues/2003-04/gt/index.php
Everything You Always Wanted to Know About Sumps, Part III
by Gregory S. Taylor
http://www.reefkeeping.com/issues/2003-07/gt/index.php