I was interested to see a strong magnet stuck to the glass of this fish tank. It attracts another piece through the glass which can then be dragged up and down the glass to clean it. The glass was about 5mm thick. I can't find any information on the strength of these commercial magnets nor anything about how field strength decreases with distance for pole-faced magnets. For a current-carrying conductor, it it is double the radial distance, halve the field strength. I think I'd need to do some experiments with this to find out the thickness of glass at which it no longer attracts.
