Bought one of these at Imagine Ocean and then won another in a raffle there. They work awesome! I had seen them in a magazine and thought they'd be crap. The magnet is super strong and flexible. It works great on the curved corners, and slides in between rocks and powerheads. I haven't scraped or wiped my glass in forever and it took it off anyhow! I thought for sure I'd have to start fresh and maintain, but nope!

A must have!

http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=20034

Does it do anything for coraline?

I had just the beginnings of coraline, so I don't know if it works on the "baked in" stuff or not. I would venture to say no.