Sounds like you're syphoning the water from the tank to the sump. Not good because when the power goes out you can't stop the syphon.
Dan F suggested raising the the filter which is exactly what I did (see my previous photo). Pump goes in the tank which pushes the water into the trash can. Trash can overflows back into the tank because it is higher than the tank. Power stop....pump stops.....nothing overflows.
Here's a diagram I did for a pond but you get the idea. The pump can be a submersible in the tank.
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