Easy to use, practical and interesting. Just screw the compensator to a water bottle, then stick it to the tank. When the water level drops to reveal the shorter tube, the water in the bottle automatically flows into the tank due to pressure difference. <br><br> When you are on a business trip or traveling, use our water compensator to avoid water shortage in your fish tank, aquarium tank, tortoise tank, or other tanks. Better than a float valve, no wiring or power required, takes less space. <br><br> Built-in noise reduction device can effectively reduce noise. (Note: not completely silent). <br><br> Screw-free, no damage to the tanks. Made of corrosion-resistant ABS material, which is also usable in saltwater. The length of the tubing (manually cut) allows you to adjust the water level. <br><br> Each compensator comes with 3 replaceable slots, suitable for 4 kinds of tank thickness: 4mm, 6mm, 8mm, and 10mm. For other thickness, you need to prepare extra gasket.