Aquarium cleaning magnets are not all made the same. Good quality magnets made of rare-earth metals in a well sealed case that will not fall from the glass when moved quickly are not cheap. I started with the cheaper units and moved up the line until I spent $100 on a Magnavore 6. I'm not saying you need to spend that kind of cash on a magnet -- but this sucker won't fall off. And yet it still does poorly on removing coralline algae from the glass. It'll do it, but it takes some elbow grease. You may find a new razor blade works much faster or an old credit card (acrylic safe too!). If you don't like the mess of getting your arm wet with a razor or credit card then look at the Penn Plax Magna-Scraper combination I use.

You need to be very careful that you do not get any sand grains between the magnets or you will scratch your glass.