This is just a belated follow-up to report success with pool problem. I found $3/gallon 12% bleach at BJ's (and see it now at Ocean State Job Lot) and have used it generously. Pool finally cleared and has stayed clear. I have tried to maintain CYA but sock approach is not keeping up. Just added DE to sand filter and the water sparkles, although this may be my imagination. So thanks for all the great advice that was both effective and economical.

Now I am trying to summon the courage to deal with the metal stains......