I use a mag float. I actually have 2, one is for acrylic (which seems to work well - can't detect any scratches). On the green spot algae, I only use the side of the mag float that goes in the water, and elbow grease (but this is also because the glass surface is curved, so I have to hold it at an angle and use just the edge of the cleaner surface. For softer algae and diatoms, the magnet is powerful enough.