[hummer]

I turn off my system as well. When I turn it off, a few gallons syphon back into the sump. To do my water change I syphon water from the main tank. Then turn my system back on so that it pumps some water back up to the main tank. When the sump water is low enough, I turn off the system again. The sump now has less water and I add new mixed water to it and let it warm up before turning my system on.