Black tank flush, again and again and again until it reads empty. I had no idea how many lumps get left behind until I had a black tank flush. Will never be without one now. Grey tanks are infamous for clogging and of course there is no tank flush, which there should be. Mine was stuck on one third for weeks and suddenly it read empty. Grey not as critical tho, cause it probably is a piece of food or soap scum. Very different with black tank as it is usually a turd. Nice to get rid of those little devils.