Can we add fragrance or essential oils to the water in the cassette, to scent the air from the water vapor?

It is not recommended to add any fragrance scent to the water as it may cause damage to the burner. Normal tap water can be used in the Opti-myst® as long as the tap water is not considered to be hard water. In the event your tap water is hard, you may use softened water or distilled water with ⅛ tsp. of salt (0.5 mL) added to the water reservoir. (The use of additional salt should only be when you notice that the unit is not producing mist as expected.)