How did you test the 'one of each' that you bought? Maybe my experiment
is
not needed!

I simply read between the lines what the Firestone guy was saying and tried a
roofing membrane. Not being certain what the manufacturing change date was, if
it was toxic it would kill the fish. I dug hole 12 x 12, lined it, filled,
added long tailed comets, they lived, spawned and have been there for 6 years.
I used the pondguard on a pond for fish that were very expensive so I wasn't
going to chance anything.
I bought another membrane a few years later. I found no difference to fish
health or liner performance.
If you go back in the archives, maybe the threads are still around.