fill your sump to the highest point you would want water to be at with the system off...when you turn the pump on it will pump water into the main tank, rising the level, making the u tube push water into the back, causing that to fill over the level and therefor drain back down to the sump. also, by filling you sump to the maximum level you would want to see it at, you'r gauranteeing yourself that if the power goes out your sump won't overflow. once the whole system is working properly, draw a line on the sump where the water level is, and that will be your max fill line, because if you ever fill it past that level and the power goes out the water that gets drained from the main tank will fill the sump and cause it to overflow...understand? I had all these questions when i started mine out, but now I got a RR tank. Thank god!