Several schools of thought on this and I doubt there is a right or wrong, I sort of believe the differences in actual pond liner v. roofing liner come down to marketing as you are dealing with virtually an identical product. You can purchase .45 mil EPDM Firestone pond liner or you can purchase .45 Firestone roofing membrane. I once recall a web site that stated something to that effect and I saved what was written. Someone made this comment, "The liner is made with say 30 steps and then they pull off a batch and that is roofing liner while the rest continues on the line and gets some extra UV protection(pond liner). It has nothing to do with being fish safe but only how long it will last in the sun. Kind of like a Chevy and GMC truck. The Chevy has to pass 15 inspections while the GMC 21. Not that the Chevy wouldn't pass the 21 but the GMC must pass all 21. So IMO the issue is really about how long will the liner last (pond vs roof) not the fish health.". This made an incredible amount of sense to me. I purchased my firestone pond liner from AZPonds but next time around I will purchase the Firestone roofing membrane from a local roofing supplier. Once I rinse off the talc, I see no difference between the two products. Furthermore, roofs are exposed to the sun so they would have to be equally as UV resistant as pond liners.
It is my understanding there are issues with using actual swimming pool liners as there are chemical additives that inhibit/prohibit algae growth. This could affect fish health and would definitely affect amphibian health. Those types of hemicals are engineered to kill so you might want to consider re-evaluating whether or not you want to use the old swimming pool liner for a stream bed. Maybe somebody else knows more about this.
Here's a link to AZPonds-
http://www.azponds.com/
You might also want to consider using old foam carpet padding under a new liner. Sure is cheap enough and no staples to ever worry about.
AZPonds is where I bought my pond liner and felt underlayment but again, next time I am going to go with roofing liner and probaly foam carpet padding as an underlayment. Manufacturer's reps can tell me there is a difference in the two but then with all the successes so many people have had with roofing liner, I really don't buy it any longer and am going for most affordable. It seems practical to me but may not to you. I wish you luck with your decision as I really struggled until I read the analogy of the Chevy v. the GMC truck.
