PVC Backflow preventor

[TRIX]

Hey. I would like to keep my return line below the water line. Is there any backflow preventors that I can use on the return line. My return line comes in at the middle of the tank on the bottom. I was hopping to do a spray bar along the bottom. But because this line won't go to the top of the tank if I loose power all the water would be sucked out of the tank and cause a major flood. Any suggestions. Thnx