Sump help

[Glennrf38]

Well I finally got it built 29 gallon tank with a 20 Gal sump using the HOB PVC overflow. Everything is working well. I tried stopping and starting the pump and it starts up right away. Tank stays full from the overflow.

Now onto the problem. When I turn off the pump, the water from the tank drains back through the pump. Ant ideas on how to stop it from doing that? is there some sort of anti siphon valve out there or what?

Thanks in advance. I can post pictures if anyone is interested.

Glenn