The uppermost tank on the machine’s front is the clean water tank. Pull the tank out by pulling down on the button at the top. Open the flap by lifting it. When filling the tank, add warm (maximum 140°F/60°C) tap water as directed by the fill line. The fill lines on the tank will tell you how much BISSELL formula to add. Lift flap, then press it into position. By aligning the bottom edge of the tank and pressing firmly until it clicks into place.