Your are correct that fish must be acclimated and not "tossed in"; however, I would like to clarify that most garden pond fish are carp. Carp by nature are cold water fish. The cold water will not kill your fish, but rather the sudden temperature change will shock/kill them. My experience with pond liner is that there is no difference. I've been building water gardens for 17 years now and have never had an issue with customers calling me because their fish keep dieing. If you ask me, "fish safe pond liner" is merely a scare tactic to get more $ out of your pocket. If there is a difference, 17 years worth of customer's wouldn't know it!