Starting from the beginning, last season I had what I thought was some type of algae. Pool store kept selling me chemicals and talking about chlorine demand, nothing worked and finally I changed the sand in my filter and all the water. It helped and pool was clear until we pulled the cover back this spring. It stunk bad and had white chunks floating around in the water?? This was the point I heard from a friend about this web site TFP. Read through pool school and water chemistry, got a test kit TF1000 and started to work. It took a while, but I got the pool crystal clear, maintaining chorline levels at about 5 to 7 ppm, PH7.5 CYA 60 and everything was great. We had some friends over and one of the children had a life jacket on which I found out later had been in a lake. I immeadiatly shocked the pool up to about 26 ppm and you may not believe it, but I could tell by the way the water looked I was in trouble again. It had kind of a fuzzy look, no longer the crystal clear it had been. Now with this being end of season and I use the 3" tablets in an automatic chorlinator my CYA has increased to about 120. Using the pool calculator I am presently doing mustard algae shock at 64 ppm and it drops to about 40 ppm over night, brushing every morning and evening. I top off my pool with well water ran through a filter, I think they call it a bobby? just so you know all the facts. My PH keeps falling from 7.5 to 7.2 takes about a week. I airate and get it back up and repeat pretty much weekly. My Calcium hardness is about 200 and TA about 120. Another thing that is wierd is the foaming on top of the water. Last year I told pool store about this and they look at me like I'm crazy. When I airate it is like the aireation brings out the algae or whatever it is and the chlorine can kill it. I get white foam on top of the water and blue greenish stringy stuff in the skimmer basket. I had above ground pools for years and never had this kind of problems, this is my first in ground, but the principles should be similar. I just can not get this stuff or algae or whatever it is to go away. I sure I'm missing something, but getting frustrated, any ideas?