It's hydrochloric acid. I don't leave it in more than about 5 minutes, helping it a little with the toilet brush, then a good flush to clear it. I have a plastic tank and diaphragm overboard discharge pump, so I don't worry too much about what happens downstream. I lube the pump annually anyway, with a good smear of Teflon grease. (on the PHII keep the grease off the pump head gaskets or they tend to squeeze out of position when tightened.)