For the best effects, it is typically advised to use distilled or purified water. Here are a few explanations:   The pollutants or minerals in tap water may accumulate over time and clog your diffuser, reducing its efficacy. Chlorine and other contaminants in tap water may interact with essential oils to change their aroma or therapeutic effects.   As distilled or purified water is impurity-free, using it in your diffuser is a cleaner and safer option. Using distilled or purified water can also help to extend the life of your diffuser by reducing mineral buildup and preventing clogs.   If you decide to use tap water in your diffuser, be sure to replace the water after each use to stop bacterial development and clean the diffuser frequently to prevent buildup.