Are you running a standpipe of any sort on the overflow? The water level in the overflow should be an inch or 2 below the overflow wall.
In a full system, if the pump removes all the water from the sump then the tank will overflow. In a sump system, the water in the tank will always stay full and the sump level will drop, usually from evaporation.
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,