The magic erasers have never scratched my tank/glass/any kind of glass or dinnerware :D Maybe you're using a different one or something with a scratchable surface/coating? I use it on my stovetop(flat surface ceramic top) and everywhere in between. Just don't use it on surfaces with paint. You'll be sorry xD I wouldn't use it on plastic aquarium decor either.
I don't recall how much they cost, but they are SUPER handy for anything...coffee stains on cups, vinyl car seats, dishes, caked on stovetop messes, microwave, bathtub, sink, LAPTOP(squeezed out really well just with plain water), you name it. I know they sell them in a value pack from costco.
But yeah, in my experience, they have never scratched the glass of my tank or the smaller thank that fred is in while the larger one cycles.