when i set up the tank it was running fine for 2 or 3 months then it started to fill up the main tank too fast so the pump was emptying the sump then then it changed and the pump wasn't quick enough to pump the water back up into the main tank .
i think it might have something to do with it being in a planted tank because some of the leaves that get broken off get caught around the intake pipe which reduces the flow into the sump.
so what i did was lower the output on the pump and then it was working fine for 5 days then the leaves must have come free and the flow picked up again and flooded it.
That is what i think because my other sump on the 600L cichlid tank has been working fine since i set it up about 5 months ago