Preformed Ponds for Coldwater Fish
If you want to keep cold water fish in a good environment then I think preformed ponds are one of the best ways to go. The advantages of a preform are mainly on the ease of constructions side, but there's also the consideration that a preformed pond is made of string, rigid durable material and therefore less subject to the possibility of leakage, leading to loss of water and stress to the inhabitants, maybe even death.
Pond liners and Preformed Ponds
Keeping coldwater fish is a nice thing to do, and if you start off with a small fish tank or even goldfish bowl indoors, then the interest will be kindled and a some point the desire to keep more or bigger fish will lead to the idea of installing a garden pond. If you are keen on nature conservation then a garden pond is a great idea because you can keep native coldwater fish, plants and amphibians. I'm not saying a pond liner is a bad idea, just that preformed ponds make a better alternative in most cases. The exception is where you want a very large pond or a special shape that isnt available as a preform.