Began having a problem with small patches of algae in a crystal clear pool. Vacuum every day but its back the next morning. Noticed very high chlorine usage after I had been adding a gallon of bleach a day. Tested CYA and black dot does not disappear at all. Drained and refilled about 12 inches of pool water twice but CYA reading is the same. Question is how in the world does CYA get so high when all I have been doing is adding bleach? We have had above average rain here in Central Florida and also my pool gets pine needles but I get them out quickly. Any help would be appreciated. I'm guessing I need to continue the drain and refill. How many times?