For tough stuff stuck to glass, I like the plastic scraper the LFS sells, but honestly the best thing I have used is a soft Mr. Clean sponge that you can buy at the supermarket. (The type that is all white without the big holes) They're cheap enough to use and throw out. If I only use the scraper, I have to clean more often. Together with the sponge, the "glass" (I have an acrylic tank) looks super clear.
The magnet cleaners do nothing but scratch the acrylic for me.