When the power goes off, water drains into sump.

How are your returns plumbed in? Many people drill a small hole in the return
just below the display tanks running water level. That way, once it drains
past the hole and hits air, the siphon is broken. The other option is check
valves, but they can stick open so you have to clean/check/replace often.

Do you have pics of your plumbing?
~John