Good topic I have been looking for the easiest and safest cleaning method, so as not to scratch the glass. But my experience has not been good:
I picked up the gray glass float magnet cleaner and my first use scratched up the glass. I only cleaned the top 3/4 since people have comment about picking up sand will scratch the glass. The pad, feels like Velcro, has nothing hard or sharp. When I look closely at the glass I can see a lot of the cleaning strokes I did.
I am not sure what went wrong? There was some (not much)coralline on the sides could that cause the glass to be scratched.
Fortunately the scratches are not real noticeable but I do not want any more scratches.
Last week I picked up a Kent scrapper with stainless blade and it works easier and quicker than the magnet, though you have to open the top and put your arm in the water.