Many people keep it closed until there is a bit in there, then open it so it doesn't get paper and gunk stuck on the bottom. We have left ours open and use a bunch of water to flush and have never had a problem. Always open the black tank first, let it drain and then open the gray water to flush out all the gunk from the black tank. We flush the black first, close it and fill with lots of water. Meanwhile we empty the gray, close it and run water in the sinks. Then we open the black once again and close it, then the gray and close it. We fill some water in both tanks, treat the black tank and rinse the hose and are on our way, fresh and clean!
