Adjusting the Total Alkalinity is likely to help. The term technically refers to the ability of a solution to neutralize acids—or buffer them. In your hot tub/spa or swimming pool water, the importance of measuring TA is only slightly different. TA acts as a buffer for the pH level in your water, helping to keep the pH level stable.
Total alkalinity is important to your water balance, the first step in your water care process should be measuring and adjusting TA before adding any other chemicals. The ideal range for TA is 125 parts per million (ppm) to 150 ppm.
When you adjust your alkalinity, add small doses, one at a time. Allow the dose to circulate before testing again. Only after your TA is in the optimal range should you move on to adjusting pH. Achieving the right TA may get your pH in the target range.
Should I use Bromine or Chlorine sanitiser if my pH is unstable?
Bromine is slightly more effective in an unstable pH environment. But this isn’t really a basis for choosing which sanitiser to use. Achieving a balanced pH water is a necessity, whichever sanitiser you use.