Even the algae scrapers that use a razor blade can scratch glass if not held at the correct angle or if the blade edge slides sideways on the glass, so that’s not necessarily the answer. That said, the tool you’re using could potentially scratch the glass also. For either the algae scraper or your leveling tool, pressure applied can make the difference. You’re thinking that wasn’t much help. Read on...
Do you really need an algae scraper? When you scrape the algae off the glass, everything you scrape off goes into the water. Some of the algae ends up in your filter media, the rest floats around and ends up on your substrate, your plants, your decorations, etc., where it can possibly start a new colony.
I’ve found most algae can be removed with a piece of paper towel. Just swipe from top to bottom. If you need to make multiple passes, grab a new piece if paper towel. Nearly all of the algae is caught on the paper towel. Compared to the amount of money we spend on our aquariums, paper towels are cheap. It’s important to use white paper towel, not the stuff with prints. Fish have been known to jump out of the tanks because they were scared by the ugly prints. Okay, maybe I made that last bit up.
The edge of a (hopefully expired) credit card works if you don’t want to use paper towels, but I’ve found that paper towels and occasionally a little perseverance gets it all off. Maybe I just have easy algae. Some people use sponges (make sure it‘s new). If you use a sponge, use a separate one for each aquarium to keep from spreading algae, and possibly parasites or snail eggs. With paper towels I don’t have those worries.
