I recently had a problem with dark spots forming at different areas of the quartz pool finish. Mainly in the deep end but some on the sun shelf too. I took pictures of it and stopped in at the store. The person at the counter tested the water which was good so he had John take a look at it. After asking many questions and spending lots of time with me, John believed he knew what caused it and showed me how to spot treat a small area and if it worked on the small area, how to treat the whole pool. Well, he was right and after treating the pool, my quartz finish is look great and back to normal. Thank you John and team for taking the time to help me fix my problem!