Do you have to buy CYA?

[rastoma]

I haven't added any chemicals in a week other than bleach and borax and I've had to top up the pool a few times (it's a small pool) and CYA has gone from 50 to 60 after a week. So if it's not in tap water how has it increased?

[duraleigh]

The test is subjective. One person will read 50, another 60. It only increases if you add it to your pool.

[Watermom]

Also, it is slow dissolving. It takes about a week for it to totally dissolve, so it may just be that when you tested and got the cya reading of 50, it all hadn't yet dissolved.