We are sorry to hear this. The humidifier will utilize one tank at a time. It will completely empty one tank then automatically begin to use the second tank. The use of high mineral content hard water may cause a white mineral residue to accumulate on surfaces in the room near the humidifier. The mineral residue is commonly called “white dust.” The higher the mineral content (or, the harder your water is), the greater the potential for white dust. The white dust is not caused by a defect in the humidifier. It is caused only by minerals suspended in the water. The HoMedics Demineralization Cartridge will help reduce the potential for white dust. The cartridge should be replaced every 30–40 fillings. The cartridge may need to be replaced more often if you are using very hard water. Replace the cartridge when you notice an increase in white dust buildup. If white dust still forms around the humidifier after a new cartridge has been installed, consider using distilled water If you live in an area with hard water or use a water softener, we recommend using distilled water for better results from your humidifier. Never use water-softening additives in your humidifier.