It is necessary to test your CYA level, because that's what will determine where your chlorine levels have to be in order to not get algae blooms. As your CYA rises, so must the minimum level of chlorine (see the "Best Guess chlorine chart" in the link in Watermom's sig). You don't necessarily have to buy a CYA test kit, the pool store can test it for you, but they're going to insist that you buy a bunch of stuff from them, most of which you don't need. So--if you can resist the pool store hype, they can do the CYA testing for you. If you can't....then you've been warned!

Next time you go to WalMart, take a look for the 5-way hTh kit--that one is less than $20 and includes the CYA test.

Janet