I really do think it has something to do with calcium too. The first year I had staining, the pool store had me dump in lots of calcium. I did a stain treatment and ended up with a pool full of milk. Then they had me use a flocking agent - and turn the filter off for 2 days. After the water cleared up I did not have any stains for the rest of the year. The calcium did drop a lot if my memory serves me right. Then I went a couple of years without using calcium as I thought it was not necessary. I had a lot of staining at this time, and learned how to get rid of stains very well!!! Then l read where a fiberglass pool needs calcium to keep something (I am not good at any chemistry as you know) from leaching out of the gelcoat, maybe cobalt? So I started using some calcium. Last year, I know I posted it, my pool was all balanced, the water very clear, but there was a greenish tint to the water that I could not get rid of. I put enough calcium to get it up to 300, and right before my eyes the water turned blue! I had no idea why or how it happened. I didn't think it was a coincidence, because it was an immediate reaction to the calcium. I did notice that when I added bleach to the water, I could see the water cloud. I think I even asked you about it. Once the bleach was circulated the cloud dissipated. Now this year it happens again - but I have not had one stain since I treated the water at the start of the season. My pool has been holding chlorine, the ph raises with the bleach so I add a puck because I hate the muriatic acid. I have not used anything else in my water and my pool is white! I have a couple of bottles of sequestering agent and some ascorbic acid sitting on a shelf - I love it. I do have the culator in the skimmer basket too - so I don't know if it has anything to do with it. We'll see next year when I open. My pool always opens with no cya, and low ph. I try to add bleach under the cover when we have a couple of warm days, but basically I am starting from scratch every year. I live in the middle of the woods so my water gets full of so much stuff I don't know where the metals come from. I also have a well and a heater- that's why I gave up trying to figure out where they came from and just concentrated on getting rid of them. It would be so great to just have a milky pool at the beginning of the summer and once cleared up not to have stains! Maybe this is it???