I wasn't sure which section to post this in. I have been doing the TFP method for over a month now. My pool has never been more clear. I have a flagstone around the waterline of the pool. It has a dark ring that has been there for a long time. Today I took a 3" chlorine tab and rubbed it over the line. I let it sit for about 5 minutes then scrubbed with a pumice stone. Very little to no change. It's right on the waterline. Any ideas on how to get rid of it? Thanks.