I use the algae scrappers and cleaning pads that say they are safe for acrylic.
The quickest way to scratch your acrylic is to catch a bit of sand or gravel in the cleaning pad. Don't go near the substrate with a pad, use a plastic scrapper near the bottom of the tank. They don't seem to catch the substrate in such a way that it scratches.
and technically most acrylic tank manufactures will tell you to use a microfiber or chamois cloth and some type of acrylic cleaner like the one posted above. they are easy to find online, even most online fish product stores will sell something to that affect.