Magic erasers work great for basic cleaning of daily growth of algae but anything harder like Coraline algae will tear them up. You will still need a scraper blade for that.
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Sand alone will have no effect on the tank but, 1 tiny grain of sand stuck in a magnet tank cleaner will quickly trash a tank (acrylic or glass). I am very careful in my glass tank but use magic erasers on acrylic.
Every tank will get scratched EVENTUALLY. There are other reasons to not run sand (and an equal amount of reasons for having sand) but Ive never heard that one.