20 Year Warranty
To begin with if your liner doesn't come with a 20 year guarantee, walk away. Walk away rather quickly. Remember that if plastic materials degrade in the sunlight (cheap ones do) then the water doesn't get held in. Ponds with no water in them are not a lot of fun and finding and fixing holes in cheap liners is a waste of time (not to mention big-time frustrating).
Fish Safe
Fish safe? There are a lot of conflicting reports on the net about the nature of pond liners. Some say it doesn't matter if the liner is guarantee fish-safe or not as they put in roof liner material and it didn't kill their fish. They are right – it didn't kill their fish.
Plastic Issues
Here's what you need to know about plastic production.
There are a lot of different "recipes" that manufacturers can use to make pond liners. Just as there are a lot of ways to make bread that taste good or are cheaper, (using a little of the cheaper barley flour instead of more expensive wheat flour - save a few cents and you can't taste the difference) there are a lot of different ways to combine products to make pond liners.
Some of these are fish-safe and some are not.
If a company finds a cheaper source of material for their roofing plastic, they will use it even if the material kills fish (you don't find a lot of fish on roofs). And product runs differ from batch to batch depending on the price of the component material. The company making roof liners doesn't particularly care if fish are killed by its product, they just want to hold water out of your house.
Cheaper Will Cost
The bottom line is that you can use cheaper roof liners and the odds are that you'll be lucky and your fish will live. Or, you could be unlucky with that batch of plastic The question is the savings worth the chance and cost of having to replace the liner?
Your call.
Here's my page on the different kinds of fish pond liners and an evaluation of each of them.
Shopping Resources for Fish Pond Liners
Fish pond liners only use product recipes that are considered safe for fish.
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