【Building a Landscape with Our Pond Liner】Premium Material & Robust Tensile Strength & Excellent Flexibility. To prevent your pond from losing water overnight, it's important to have a barrier between the water and the soil. Using 20Mil liners 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.