@Gemma here is some info I pulled when looking for my own pond liner. EDPM liner is made from recycled tires, and other ingredients, so each batch is different and made with cost efficiency of sourced materials to produce it in mind. Some will be fish safe, but others won't. So you could get a roof liner that is fine, or one that will kill the fish, as the materials used were not tested for fish its hit or miss. My home and garage is really old, as far as I'm aware the liner on the garage has a heavy dose of algaecide or fungicide added, can't remember which. Depends on each company and what they order from EPDM suppliers. Here was something firestone I saw when researching liners. Hope you can read it.
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