I have posted my learning process elsewhere. Changing to BBB from Trichlor pucks. In spite of high CYA, 160, in spite of my ignorance for the past 5 years since I bought this house with pool, I have not had algae that I know of. But now since this site has made me more anal about my water I have a question. My pebbletec pool shows no signs of algae, not at waterline or walls or floor. Nothing else in pool except a light, no ladders or slides. But what I do have is 5 or 6 dark spots about the size of a quarter, one the size of a small saucer, on the bottom of the pebbletc. They do not respond to nylon brushing. Any concern it is algae? Since I have never had it I almost do not know what to look for. Hopefully having kept my FC at 10-12 (luck only) kept troubles away.