You need to avoid adding anything the pool store tells you to add EXCEPT what's needed to maintain chlorine levels OR to keep the pH above 7 or below 8.

And you need to learn to test your water accurately. A few pool stores have reliable testing, but most don't. The only way to you ca tell the difference, is to test your pool accurately yourself . . . but once you can do that, you don't need the pool store!

Test kit info page linked in Janet's sig (above).