A good place to start is to determine the amount of turnover you're after (e.g. 10x per hour, 20x, etc.) and multiply that by your total system volume to get an idea of how many GPH you need from a pump.
Also, I could be wrong on this and would like to know for sure, but I don't think that there is head loss on a CL system because its all from within the same body of water (e.g. the display) so there is no gravity differential . . . again, not sure.
right now i have 2 Maxi-Jet 1200s which are 295 gph each. i also have a Mag 5 for my sump return which is roughly 300 gph after head loss.
i want to replace the Maxi-Jets with a closed loop and at the same time slightly reduce the flow (a little too powerful for the fish imo). the closed loop will have 2 retruns using 1" flexible tubing. no SCWD.
a pump around 600 gph or less would be good. i am looking for one that is quiet and reliable. no leaks!!!!