There are two valves on that return line loop @Guy Cavallero . In normal operation the bottom valve is closed and forces the water over the top side of the loop. The centerline of that top pipe is where the water level will always stay in the buckets, the height of the loop is determined before construction. In the event of a pump failure or power outage the buckets can only flow back to that level, hence never flooding the reservoir and always having full buckets even if there is no circulation. The bottom valve is used for reservoir changes when I want to empty the buckets. In one of the pictures earlier you'll see a valve on the outside of the reservoir. I hook a hose to that and run it to the floor drain next room over from where the tent is. I open that bottom return valve which lets everything flow back to the reservoir and open the reservoir valve to run everything to the floor drain.