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Most zippers these days are plastic, except for the heavy-duty ones. Is there any way you can contact the seller and find out? I would think if they were specifically made to hold filter media in an aquarium, they would be of entirely safe materials. Then again, it's best to find out for sure than to assume the obvious. If in doubt, and you can't find out, perhaps your local LFS has some with ties instead of zippers.
20 gal. high: planted; 5 white cloud minnows, 4 golden White Clouds, several RCS, 2 blue shrimp, 5 Amano shrimp, several snails; Azoo air. 65 gal: planted; 6 rosy barbs, 6 yellow glofish, 3 red glofish, 3 zebra danios, 5 white cloud minnows, 3 dojo loaches, 6 crimson spot rainbow fish, 12 large Amano Shrimp, several snails; AC110.