Paulkelly555

Recently, I have noticed my Red sea Reefer 750 XXL tank is filling up and water not going back down the downpipe. I set my return to the lowest setting. And also opened the valves full. Last night, the tank literally filled up and overflowed and flooded my house. I have now switched off all pumps - and place heaters directly in the main tank. I have tried a few times putting pump back on - and yet still same issue. Any ideas?