To prevent your pond from losing water overnight, it's important to have a barrier between the water and the soil. Using VEVOR's 45Mil Pond Skins can prevent water from leaking out of the bottom of your pond. A koi pond liner is essential to keep dirt, debris, and contaminants away from your fish, which helps them stay healthy. By ensuring that your fish have a clean and safe environment to live in, you can provide them with a happy and healthy home. VEVOR is here to help! | Vevor | | Pond Liner, 15 x 20 ft 45 Mil Thickness, Pliable EPDM Material Pond Skins, Easy Cutting Underlayment for Fish | Black | Best Buy