Well, I'm really frustrated. I was getting a lot of algae because ny CYA levels were so high I could not keep the chlorine levels high enough. So I drained the pool to about an inch below the bottom step (15,000 gal in ground, fiberglass). Refilled and CYA still tests way over 100!. Black dot dissapears about half way between zero and the 100 mark. What else can I do? I hate the thought of having to drain and refill again.