All About Flexible Pond Liners
PVC and EPDM pond liners are by far the most widely available. PVC, of course, stands for polyvinyl chloride. While that doesn't sound like something you'd want to expose your fish too, PVC is quite safe for all aquatics. EPDM stands for ethylene-propylene rubber. What's the difference? Well, PVC pond liners are a bit more flexible than EPDM ones, but EPDM liners can handle temperatures down to 40 below, whereas PVC liners shouldn't be exposed to much more than 20 below. Both kinds of liners have very good seal capabilities (so if you need to “glue” two bid sheets together, either PVC or EPDM will do fine), but EPDM tends to have better UV resistance, and is a bit less likely to tear. That said, you'll still want to limit the sun exposure for any kind of liner you get. Finally, PVC liners do have a reputation for having a shorter lifespan than EPDM liners.
Given its slightly extra strength and ability to withstand long-term exposure to sunlight, EPDM liners are a better choice. But if all you can find is a PVC liner, go ahead and use it with confidence. The best-selling brand of EPDM liners is Firestone, which offers a 20 year limited warranty for their 45 mil and 60 mil liners. The difference between 45 and 60 mil is thickness - 60 is thicker than 45. Both weights make an excellent liner.
You may also come across butyl liners, which are comparable to EPDM liners in strength, flexibility and UV resistance. Many long-term pond builders say butyl liners considerably outlast even EPDM liners. They do cost more, however, and are harder to find. If you're satisfied with a 20 year life for your liner, EPDM is probably the best compromise of quality and cost.
Of course, if you want your pond to last a lifetime (or longer), you'll need to upgrade to a full-fledged concrete pond. The cost for a concrete pond is at least three times what a flexible liner will run, but if you are going to be keeping $10,000 worth of koi in the pond, it may be worthwhile to simply hire a professional koi pond builder and build a first class pond.
If you can't afford a concrete pond, but still want your flexible pond liner to last as long as possible, do buy pond underlayment. The store you buy the pond liner from will almost certainly also sell underlayment. If the extra cost is a prohibitive, you can also use heavy grade roofing felt, or even carpet or any other kind of padding. Just be sure it won't disintegrate in 20 years, and - ideally - that it is non-toxic. Just in case a leak does happen, it would be a shame to poison your fish with the underlayment.
Leaks? Yes, they can and probably even will happen over a few years time. Do yourself a favor and buy one of the sealer kits available BEFORE you need it. That way when the leak does happen, you won't lose precious time (and water) going out and getting the sealer kit… or worse - waiting for it to arrive in the mail.