ok if you have a glass tank, an overflow is used to siphon out the water out of the main tank, into the sump, without fear of it flooding the floor during a power outage. i am sorta particular to the CPR overflows, they are awesome and really quiet, way more than Amiracle. So anyways, if you were to "rule of thumb it" and want your tank to cycle at 8-10 times per hour, then you would be looking at flow rates from 600-750 gallons per hour. one thing that you wanna make sure of is that your overflow is capable of flowing more water than your return pump will provide or else your floor will be wet. if i were in your case i would look at something with a flow rate of around 800 gph. I just found a CPR CS100 for about 90 bucks.....i would look a little harder if i were you, but thats probably average. keep in mind, this is a very important piece of eqiptment so you really dont wanna DIY it. nuff info?