In Ultrasonic Cleaning Process it is always recommended to use DI water as the main cleaning fluid as it is an ultra pure water and does not have minerals or other contaminants in it. It provides an excellent bridge for the detergent and there have no chance of depositing minerals on the parts. It can be used in the rinsing of the detergent and will provide stain mark free drying.
DI water is simply water that has most of the minerals and dissolved ion content removed. It is an ultra pure water. The cleanliness of the water is rated as its resistance value. The rating is in M.Ohms.