Have had the pool for 15 years ,21,000 gallon inground with vinyl liner, and de filter. This spring was typical after removing the LoopLock cover..dark green. After pounds and pounds of shock, alkalinity raiser, algecide, eight days of backwashing, dismantling the filter to clean it, and adding well water to keep the skimmer going, you could finally see the bottom. Thursday night it rained and in the morning the water was BROWN. Took another sample to the pool place and was given basically the same chemicles again 25# Balance pac 100, 4# shock, algecide, etc. As of today, no change, however the filter pressure does not rise, there is no sediment on the bottom or in the filter so there has been no need to backwash. It just stays brown. Someone suggested iron in the well water but I've been topping it off all these years.