I used to have that problem (ph rising over time) when using only liquid chlorine as my sanitizer/oxidizer. Since the liquid chlorine we use in our pools is inherently alkaline, its continuous use without taking counter measures will result in increased ph levels. As others have indicated, adding acid will help keep your ph in check. However, I found that by alternating the use of liquid chlorine WITH trichlor pucks, I can keep my ph within acceptable range. Since I have had trouble maintaining an adequate level of cya in my pool in the past, this was a good solution in my particular situation. Besides, it was getting old lugging all those bottles of liquid chlorine from the store. Now, I use maybe less than half as much liquid, and the cost differential between the two is insignificant.