Relax pH Minus/Reducer - 7kg/25kgpH minus reducer is an acid, usually supplied in liquid or granular form, used to lower the pH of pool water when levels are higher than 7. 6. Chlorine works best as a disinfectant when the water is in the pH range 7. 2 7. 4 Relax pH minus is sodium bisulphate, and can be added by sprinkling over the pool inlets or by adding into a skimmer, for rapid mixing with the pool water.