A lot depends on your method of dumping. If you have a macerator, which we do, any single ply will work. It is important that the tissue dissolve or it can get caught in the blades during the dump cycle...IMHO, even if you dump with the stinky slinky, I would use tissue that dissolves quickly as Charmin or Northern etc. may get hung up on the sides of your tank and create a real mess getting it "unstuck"...then again, I've never really done a close inspection of the inside of my black tanks...ever....:W......Dennis