First, you would need to determine how much headloss you're going to have, ... pumps lose GPH as they pump up vertically so figure out how much this is, accounting for any bends (Which can further reduce GPH) from where the pump is to the re-entry point in the tank. "Head" is the height the pump has to push, and "headloss" is the reduction in GPH at that point. Such as, if the pump is exactly four feet directly below the re-entry point, then there's a four-foot head with a four-foot headloss ... four feet below but with two 90 degree bends with a one foot horizontal run may equate to a six or seven-foot headloss.
Then, most pump manufacturer's websites, some aquarium equipment suppliers, and even the boxes the pumps come in, will usually have a chart specifiying the actual GPH at particular heads.
You want your actual GPH at the re-entry point to be close to, but not exceeding your drain capacity. I have also heard that it's best not to push an overflow to its maximum capacity, around 80% or so preferable.
IIRC, my Mag7 with a hair over four-foot head is putting just under 500GPH into my tank, pretty close to 80% capacity of my 600GPH overflow.
... can someone help!
