Polyethylene Liners are the best choice for your Natural Swimming Pools/Ponds as they are durable, long lasting, and are safe for fish and plant life. The liners we use are a 40 mil, they are super tear resistant, can handle extreme cold temperatures, and are expected to last up for 50 years if they aren't directly exposed to UV light. Now these liners do well in UV light conditions (sunlight), but to prolong the life of your liner it is best to cover any areas that would receive direct sunlight.
Polyethylene liners are ordered by a project to project basis, this way you can get a liner that is custom fit to your pond and the pieces are seamed together at the manufacturer ahead of time. Head over to the Watertight Liners page to get a quote for a liner based on your specific NSP project needs.