Hi Marie, (If you want to move this to the stain side --- go ahead)

Like I said to Richard --- It's a mystery!! I was just getting ready to post on the stain forum --- when I came to clip the thread URL --- Here you are!!

I keep trying to think what the cause is but obviously I'm not getting any where. Over the last 4 years we have tried all the JUNK (before we found this site) including Nature 2, Pool Genie, various sundry of cleaners and chlorine mixes etc. etc. so who knows what's left hanging around from the past. That said --- none of the "junk" has been present this season (except for residual).

As mentioned I'm dropping the Cl etc. to get ready for the Ascorbic acid which will get followed up with the Chelating agent --- Hopefully all without going to green, pink, yellow or black. I'll keep you posted.