I wouldn't worry about it on a glass tank as much as acrylic, you could also wrap a piece of fine mesh or a fish net to 'pad' the magnet a bit. The only other caution is that rare earth magnets grab eachother hard, if you don't have a good hold of the one on the outside glass when you are getting close to putting it on, the magnet could slip out of your hand and slam into the glass, cracking it. People have had fingertips sliced off by playing with those magnets as the finger got caught between two magnets as they pulled together from a distance.