Why would you turn off the sump? The sump is there to keep the tank water level constant, among other things.
I syphon water out the top of my tank until I almost drain my sump (You can syphon out from anywhere, I am just stating what is easiest for me), then pour new water into my overflow (you could just pour into your sump) until the sump is full again. That way, water level in the tank never goes down and sump/pumps stay working as long as you leave just a little water. I have even drained my sump completely for bigger water changes, let the pump run "dry" for about 30 seconds while I ran and grabbed water to throw back in. In your tank though, draining the sump would be a big water change, so you probably will never have to do that.