All manufacturers recommend the use of distilled water with CPAP humidifiers. Still, residue may build up in the water chamber if it's not properly cleaned on a regular basis. Each morning, empty any leftover water and let the tank air dry. At least once per week, wash the water chamber with warm water and mild soap. Let it air dry completely before reinserting it in the device.
Periodically, or if you notice residue buildup, fill the chamber with a solution that's one part distilled white vinegar and two parts potable water. Let it soak for one hour. Rinse with potable water and let it fully air dry.