I maintain a pool in which the fill water is from a well which puts lots of metal in the pool. So much metal that the pool turns green whenever a few inches of water is added, and then clears up after a few hours. When i started servicing the pool the cya was up around 80, a few months later i tested and it was around 40. I didn't think much of it at the time because nothing was wrong with the pool, so i figured it was a bad test. About a month ago there was some algae in the pool and i test the cya and there is zero. I brought this up to the homeowner and she said oh yah this is normal, my old pool service would charge me yearly for more cya. This seems really strange. Is it possible that the metal is getting rid of the cya? There are no leaks in the pool. She has recently put a metal trap on her fill line, but it hasn't been long enough to notice whether or not this stops the loss of cya.
Another pool, i did a stain removal and got the guy a vacuum, tested his water, the cya was over 100. I told him he needs to quit adding tablets and only add liquid. Anyway he calls a few months later says hes having problems and that the chlorine is only lasting a day, i go and test the water and theres no cya. The pool wasn't drained, and no leaks.
Both of these maybe had a small amount of algae, but never a swamp, and it seems never long enough for algae to consume the cya. Any thoughts?