Thank you so much! Will purchase a new test kit today. The pool is in-ground vinyl, 16x32, about 20,000 gallons. We've had it for 7 years and have NEVER had a problem. I don't think we've ever even used the vacuum hose until this year. The pool navigator has always done the trick before. The pool store folks say to change the filter sand every few years-but I know lots of people who never have. We haven't. Maybe that's the problem. Whatever it is turns white when I shock the pool but always returns through the jets.