Mag-Floats scratch the heck out of glass and acrylic. I despise those stupid things...you get one little grain of sand in there, or coralline algae, or those little tube worms like to live on the freakin magnet, and you get a scratch! I still use mine on my 33g though...now that it's scratched to heck it doesn't matter!
I prefer the Kent ProScrapers.