Glass magnets are a handy tool with fish tanks. They make glass cleaning chores much simpler. These probably are not safe for acrylic tanks. For cleaning acrylic tanks use an old credit card or some other plastic scraper.
I have several sets of magnets, one set I use to hold Nori and other leafy foods to. I just rubber band the food to the magnet and then I can position it easily and not worry about the fish pulling it off the glass like the cheap suction cups food holders.
These Tunze magnets were kept in the box until needed. If the coralline was to thick for my magnet to remove then I use a razor blade. My Tunze Magnets are part# 220.54 and the manufacture claims they are good for glass up to ¾ inch thick.
After using the tunze magnets after a while I felt they were not strong enough for my glass. I've since sold them off and upgraded to a Magnavore VI magnet.