I would do what it takes to reduce the drain back into the sump. Check valves are notorius for failing and at the very least once you get a little algae/ bacterial slime growth in the lines they will leak down anyhow.

Is the water siphoning from the return pump outlets? If so bring them closer to the water surface. Or is it just volume from the display and the sump water level is too high? If so another sump or some mods to your sump may be in order (preferably another guy).

drill some holes just under your waterline in your return facing down once water lowers past that point....syphon break:):)

i have its still not enough...