Salinda,

When you had CH of 525, TA of 130, and the pH was rising and hit 7.7, then your saturation index was around +0.6 so slight scaling was possible. With your current TA of 90 and a pH of 7.5, your saturation index is a much more manageable +0.21.

If you use a non-acidic source of chlorine such as chlorinating liquid or bleach, you can lower your TA even more if you want to, even to 50 ppm, and this should further reduce the tendency of your pool's pH to rise. You'll probably find that something like 70 ppm will be fine.

Think of this as an advantage to having a higher CH pool -- you can have a correspondingly lower TA and have calcium carbonate saturation in balance and be even better off in terms of having less pH rise.

Richard