pH is the most important factor in controlling your pool-spa water balance. High pH results in cloudy water, staining, scale, and filtration problems. Low pH leads to corrosion of metal pipes, etched plaster, rapid loss of chlorine, and irritation to swimmers. This product is designed to prevent the ph and alkalinity in spa water from bouncing around. The product should be added to a freshly filled spa after the ph and alkalinity are initially balanced. Dosage rate is 1 Qt. per 500 gallons.There is a 3 units minimum, if this is the only item you are ordering.