From the horses mouth, Via email that is.
"Hi Tim,
Thank you for contacting us. The plastic coating we use on our magnets is polypropylene, which is often used in the food industry. This coating should be inert in any aquarium. If you have any other questions, please do not hesitate to ask.
Best Regards,
Kevin Stayer
K&J Magnetics, Inc.
www.kjmagnetics.com "