Always ensure that the Alkalinity of your Spa water is within the acceptable range before adjusting the pH. Trying to maintain a stable pH in your Spa will prove difficult if the Total Alkalinity is either excessively high or low.
Total Alkalinity - Minimum 100ppm, Optimum 140ppm, Maximum 160ppm
If your Spa water has a low Total Alkalinty (TA) it will have a very little buffering capacity and it can cause the pH to change quite rapidly from one level to another leading scale to form, corrosion of surfaces, eye irritation and discolouration of your Spa water.
If your Spa water has high TA then this will have too much buffering capacity and could cause cloudy water and other pH related problems.
To raise the TA Sodium Bicarbonate should be added to your Spa water. When trying to raise the TA, the water may be cloudy for a short period. Therefore it should be adjusted at the start of a filtration cycle.
To lower the TA Dry Acid (Sodium Bisulphate) should be added a little at a time at once and in one place . Test regulary during the addition; do not add large amounts of Dry Acid.