I use one on my acrylic tank with a piece of tailor cut micro-fiber cloth embedded with Novus acrylic polish number 2 or 3 depending on need. The polish stays on the micro-fiber even under the water only bleeding slightly but never hurts my fish at all and polish/cleans the glass very well. However only the medium or large mag cleaner has enough strength to firmly hold "thin" microfiber cloth sandwiched on the face of each magnet to prevent the scratching of acrylic and hold polishing abrasive in the micro fibers.I got one and was not impressed. It was supposed to be "acrylic safe" but scratched hell out of the SO's tank. (lfs guy says that many tanks called acrylic are actually just plain ol' plastic, so this may not have been entirely the mag-float's fault.) And it only cleaned a little strip like 1/4 inch on each pass, so it was a substantial amount of scrubbing to clean even one wall of a 12g.
A fuzzy plastic scrubber pad works much better, imho, besides being cheaper. Yes you get your arm wet. /shrug