check valve?

You don't want to use the ones with metal parts in them. You can get the correct ones ( swing check valve type) from marine depot and I believe BRS also carries them. FYI you will never get a 100% seal in a saltwater aquarium due to deposits/crustations eventually forming on the seals and internal parts. They will prevent the initial rush of back flow but if the power is out for a long time (hours) the water will still drain back to you return output level. Your sump design needs to be able to accommodate this.

 

Jaime is 100% correct!

I like the BRS ones.

No matter what you use, they will restrict the flow somewhat. The BRS not nearly as much though

 

I have the $15 one from Bulk Reef Supply (BRS) and its been great so far. No metal parts in it, clear plastic so you can see inside and you can replace the gasket in it too. They don't have anything at Menard's that compares from my experience.