I know it might be very late, or you have probably tried these as well, but this type of check valve I used for my ponds suction hoses end. This only lets the water out of the pond, and when the electrics are out the water pressure closes the tube so the water does not escape back into the pond.
You might be able to use this upsidedown,on the base of your nano.So when the electrics are out the water pressure would stop the flow.
But you will have to fit the power head outlet completely to this valves inlet.
