[Dentoid]

Your sump should be big enough to accommodate the excess water draining from the overflow once the level of water in your display falls below the overflow level. In the event of a power outage there would be no water being returned as the return pump would stop working. In fact no control valves should be placed on the draining plumbing.

Control valves should be reserved for the return plumbing to control how much water is being returned to the display and/or regulate flow to the fuge.