I have had no trouble with my mag float on my 5.5G nano. In my experience, the only time I have scratched glass was when I would clean down by the sand bed, stirred it up a bit and sand pebbles got stuck in between the brush and the glass. This has happened on my 55 but never on my nano. I think its because the magnets in the larger floats are very strong, making it easy for sand in there to scratch the glass. But the small ones have comparitively weak magnets in it, so if sand gets in there, it will most likely fall out or at least not be pushed against the glass so hard. This is the way I see it in my experiences anyway. Take it with a grain of salt. Ultimately, when you use a mag float, just pay attention to what is going on. If you hear a high pitched screeching noise, dont keep moving around, take it out and inspect it for sand pebbles.