Free GFO.
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
It's hard to say if rust from ceramic magnets will be problematic. I recall two little fishes nori magnet clips being reported to hurt tanks as they commonly rust, but not sure if that was coincidental or not. GFO is rust and there's no way we know how pure it is, so given the extensive use of GFO, I'm going to assume rust is perfectly fine in a tank. I think GFO may be the greatest rip off in the world considering where it is probably sourced and what it is, but hey, it works. Nevertheless, I would still remove a rusty magnet. Contradictory, I know. Hahaha