sumps using hang on overflow

[mark]

Should find a pump factoring in head loss that will match to your overflow or the flow you're aiming for (eg pump rating - head loss = overflow rating). Unless you have plans for teeing off the return for some other function (Ca reactor, phosban reactor or such) you're really just buying a bigger pump for nothing.

If planning on relying on the return pump for circulation only, 600 gph on a 100g not that much turn over. If the return pump is only to cycle water through the sump and have powerheads, streams or a closed loop for display, 600gph should be enough. Haven't played with hang on overflows but would imagine more you push through them, noisier they may be.

Should be using a ball valve and unions though to allow to remove pump for maintanence or repair if pump's to be external.