Moved to hardware forum:
Originally Posted by MorayMadness
it would empty the sump gradualy and starve the syphon.
That's contradictory. If your sump was emptying out it's a sign your overflow can't keep up and you need to restrict flow more. Either that or you overflow simply lost siphon or got clogged somehow. Did you have an overflow of the main tank?
What is your overflow rated at for GPH
? If not sure how many holes, how large are they, and is it an internal or external overflow. The Rio 2100 is rated at less then 400 GPH
with a 4ft head so I don't understand why the overflow couldn't keep up.
You need a pump that either under performs the overflow slightly or you need to T off your return with ball valves and have one go back to your sump to control flow. See photo below (which is also a link)