The best pond liner material for a fish pond is reinforced polyethylene (RPE), as it is flexible, durable, affordable, and fish-safe. RPE's unique construction makes it more resistant to damage from punctures and tears than its competitors, as well as remarkably flexible, strong, and lightweight.
Many RPE liners are ANSI/NSF 61 certified for potable water storage, guaranteeing that no harmful chemicals will leach into the water over time. This is vital for the long-term health and safety of your fish. While some EPDM liners are advertised as fish-safe, the safety standards are determined by the manufacturer and are not consistent or as stringent as ANSI/NSF 61.