PVC Backflow preventor

[MikeP]

My 190g has the same thing. Originally I thought of doing what you are thinking of. For fear of flooding I had another return drilled (through the back on the opposite side of the overflow). That bulkhead is still below the water line but in addition to a check valve I designed my sump to be big enough to handle the additional water if I did lose power. What I plan to do to do with the bulkhead in the bottom of the tank is run some PVC strait up and just above the overflow so that if my overflow ever got clogged it would act as a back up overflow.

HTH