Now, you are all set up at your camp site and you have to decide whether to close or open your "dump tanks." Mickey at Pismo Coast Service advises, keep your black tank closed and your gray tank open. With your black tank closed you can put the necessary chemicals in that tank and they will work to keep it treated. Mickey says, when the black tanks show 3/4 full, then dump them, pour a good deal of water in them to flush them out, th...