I have a mighty magnet, I think the FS-6? Can't remember right now. It is top notch, highest marks.
I also recently picked up a tunze Strong +, and that actually works too.. I was a little skeptical, but it does work just fine. Its pretty good for the coraline. I left the styrofoam bit on the inside portion, so it would float up, but took it off on the outside, because it was very slightly scratching the acrylic. No such problems with scratches on the inside though. Been working on my troublesome coralline for a bit now..