there is the algae free brand magnets, and the magnavore brand magnets for some of the stronger ones out there, I have used the hammer head float version by algae free, and am currently using the great white by algae free, amazing, actually right now too amazing, on 3/4" glass or acrylic I have to use 2 hands and all my body weight to move the magnet, I have it because of some larger tanks I used to service and some larger tanks of my own... on 1 1/2" glass it is still very firm and you have to use some strength to clean with it...

for smaller glass tanks the mag floats work good