The easiest way is to continue adding pH Up. This is generally fine because the additional elements that are added are potassium ions. Potassium is frequently the highest element in hydroponic nutrient solutions. Sometimes pH crashes because of the presence of a large amount of microbial activity in the nutrient solution. This is usually a result of poor maintenance of the system due to infrequent nutrient changes or other stresses. The best way to avoid this scenario is to keep a clean system with adequate nutrition.