So when you turn the return pump off the sump overflows? If you haven't already done this to set your max sump water level first make sure the return pump output is at the surface or there is an antisiphon hole in the return plumping just below the operating water level, this insures only the water above the wier on hte overflow will drain back to the sump. Be sure to have the anti siphon hole in a location where splashing isn't an issue when water is first turn on. Turn off the return pump. Keep an eye on the sump to be sure it doesn't overflow and be ready to remove water from the sump. With the return pump off and the water level at the overflow weir so not water is draining and the water in hte sump/refugium is about an inch below the top mark that water line ("Full, Pump Off" for example). Then turn the return pump on wait a few minutes for the flow from the DT to the sump reaches normal flow then mark the water line in the sump ("Full, Pump On" for example).