My problem was the same, only to find out that i had the musterd problem, a fine powder like form of alg. LOOK IT UP! It made me drain, raised a phosphate level up and down trying to fight it. SHOCK< SHOCK< SHOCK. the problem is when you get it you have to hit it hard. try using yellow out or yellow treat. then flock it twice and vacum to waste. Follow the directions for yellow out and hit it hard with shock and make sure you raise your ph to 8.4 befor doing the process of yellow out. I would even hit it a third time with in 2 monthes. keep flocing and vacum to waste each time! always check ph and chlorine levels. When you vacum to waste, and readd tap water, the ph goes down. The ph, low or no phosphates, and chlorine are very important factors in killing the mess in a musterd infested pool. hope it helps.