Unstable Alkalinity

[malenart]

I have an in-ground pool (specs below). I use baking soda, murriatic acid and liquid chlorine to keep my pool in balance. It is 7 years old and historically the alkalinity has been very stable at 100 -120 but the PH has always run high ever since the pool was new. Historically I have acid weekly to keep the PH at @ 7.2 This year it has been an ongoing effort to keep my alkalinity from dropping every time I add the acid. I add the acid by diluting in water and spreading it across the surfact of the water. After 3-4 weeks of adding acid at the rate of 1/2 to 1 gallon per week, the alkalinity drops to 40-60. I then bring the alkalinity up to 120 using baking soda and the cycle repeats itself.
Unfortunately I have very hard water and I accidentally let my calcium hardness get out of hand forgetting that evaporation causes the concentration of minerals...stupid...I know. CH is currently > 600 ppm. I am waiting for the fall to drain my pool and do and acid wash which will get my hardness levels back in line. In the interim I am trying to keep my PH on he lower end 7.0-7.2 so as to stop any calcium build up on the plaster and tiles. Could the high calcium hardness be causing the inability to keep the alkalinity unstable? If not, what else could it be?

Thank you for all your time and efforts!

20,000 Gal gunite pool with dark gray plaster
Southern California
Pentair DE 60 sq' filter
Pentair variable speed pump
Taylor k-2006 FAS-DPD Chlorine test kit.

 

[Donldson]

There's really no reason to keep the TA at 120. TA that high is good when using acidic trichlor tablets to chlorinate, but when using liquid chlorine or an SWG that TA causes excessive pH rise in most pools.

The best TA for a pool is the one where your pH is relatively stable, and that can include 60 and sometimes lower. So let your TA drop and it will help stabilize your pH.