When I teach a CPO class, one question I always ask is “Can anyone tell me what the most important chemical reading in your water?”
What do you think it is?
About 90% of the class will jump right in and tell me that chlorine is the most important; I have heard Total Alkalinity many times, almost as if it is the cornerstone of levels in the pool.
Fact is, pH is the most important reading. By far!
For the simple reason that the level of pH drives the effectivity of chlorine; as the pH level goes up the effectivity of chlorine goes down, and vice versa. Those of you who are familiar with ORP have seen this; probably many times. The higher the pH, the lower the ORP. ORP measures the effectivity of chlorine, so the higher it is, the more effective the chlorine is at doing its job.
Keeping this in mind, the key to running safe and healthy water in a pool is to stabilize your pH first, which is why most pools now use a chemistry controller because pH cannot be “set” it is constantly moving up and down the pH scale, however small or large those fluctuations might be.
Note that you cannot add acid or soda ash to a pool from time to time and expect that to allow you to have healthy water, the only safe approach is to install a reliable chemistry controller. Period.