Not to counter any of the above suggestions - but one other thought for you to consider. Sometimes one might perfer not to cut back on the movement of water and/or increase back pressure on the pump (for a variety of reasons) via a throttling back ball-valve. In such cases you can cut back on the flow rate through your tank but still circulate at 600GPH by using a wye or tee fitting on the output of the pump and send one up to the tank and the other back into the sump.
This will continue to turn your water over at the 600 GPH
rate, double filtering part of it and single filtering the other. The overflow will then only handle the portion of the return which is sent back up to the tank.
You can put a valve on the return to the sump to adjust the ratio of how much goes to the sump and how much is returned to the tank, or you can use plumbing sizes to regulate this in a fixed fashion (i.e. 1" output, split 3/4" goes to tank and 1/2" goes to sump).
Typically the greater flow rate is preferable for a variety of reasons including increased oxygenation.
Just some FYI