I come across this all the time "I've been using pucks for years and my water is perfect. I never test anything but chlorine and pH". I say "Great!" After all, until they have a problem, it is great. And, until they have a problem that their pool store and conventional wisdom cannot solve, I have no effective argument for them--they have evidence of success.

When they get the algae bloom and they can't clear it, then they start to listen.