I had an Iron staining problem when I opened my pool, I added shock to the water it was crystal clear but the whole surface turned a med yellow color , I used ascorbic acid and it cleaned right up balanced the water shocked it again it came right back , cleaned it a second time and realized that the iron is back in the water not on the sides how do I remove the iron from the water it was hiding in there at startup I have a 15000 gallon inground fiberglass pool with a 300lbs sand filter I am not putting any chlorine in there until I can remove the iron?