Did any snales get into your overflow and work it's way down?
You should get a Aqua Lifter Pump.
Continuous Siphon Overflow
I am a person who needs pictures. Got any?
I am a bit confused on why do you have 2 pumps? The overflow should not have a pump unless it is something like aqua lifter pump. All you need is a return pump. If you wanted 2 pumps(not recommended) then you do not need an overflow. I do not know...perhaps I am going about this the wrong way?
Zak