As mentioned in another thread I gave this a try and am in awe. I put this small bladed magnet on the back of my big one on my thick glass in the 230g and it of course jumps around a bit. But all my glass was sparkling-ly-free of coraline in minutes. For a nano, you could just use it as it. Two thumbs up from me.
http://www.aquatichouse.com/Maintena...MagScraper.asp