Return pump should run no faster than what your overflows in the tank can handle in terms of draining into the tank.. otherwise, too much water goes up into tank causing overflow of the tank...
Now, there are variables to consider such as head height (distance from pump to plumbing output - if sump is under the tank, then head is only 3.5 - 4 feet. So you're pump flow will go down a little because of that... in addition, I recommend having a
ball valve on the output of the return pump to throttle down flow a little... this reduces flow a little too...
I think a pump like a mag 7 should be fine if located under the tank... I have a mag 7 on my return for 75 and it's throttled down to about 1/2 what it can pump because my overflow (rated 600 gph) cannot keep up... a new mag 7 pump is in the $75 range - via online retailers... this pump should be used in sump - not external cause it had leaked on me... However, I've run mag 9.5s external without a leakage problem...
Note with
mag pumps - they put out a lot more flow if you double the plumbing size on the output end... so for a 3/4" output, use 1-1/4" plumbing to get it back up to the tank...
I've had 5 mags running simultaneously at one point - return, skimmer, refugium, closed loop and water mixer... they do warm up the water - which can be an issue in the summertime if you have a few in your system
hope this helps..
