In very basic terms, alkalinity is the opposite of acidity. The balance between acidity and alkalinity is called the pH balance. Water with a high alkalinity has a characteristic "soft" feel to it.
pH can be measured with a simple kit, available at most pool supply stores. Follow the instructions on the pack. A healthy pH balance for a pool treated with activator D is between 6.8 and 7.2
If the alkalinity of your pool is too low, you might not notice any improvement in water clarity even after adding activator D. When an alkalinity boosting product is added to the treated pool, the water will clear before your eyes as if by magic!