A magnetic aquarium cleaner has two enclosed magnets with cleaning pads on the inside. Move the outside magnet, and the wet side magnet follows along, scraping off green film algae from the glass. On most models, like the Mag Float, the magnets float if they get separated, making for easy retrieval. Some aquarium magnets even include scrapers to get rid of hard coralline algae. Replacement pad kits can be purchased for worn pads.
Be sure to buy the right size aquarium magnet. Measure the thickness of your aquarium glass to ensure proper adhesion and strong scrubbing action. Too small, and the magnet won’t attach. Too large, and the magnet won’t move.
If you want to know how to clean algae off acrylic aquarium sides, just be sure to buy an acrylic safe algae magnet with softer pads (and be sure gravel doesn’t get trapped in between the pad and acrylic).