When used properly, they definitely don't scratch the glass. And nothing more satisfying than a brand new blade cutting through that algae like butter. The fish like it, they try to eat the scraped off algae that floats away lol
But since it's an algae thread....
I have that filla"mental" green invading sticky string thing...
That compares to a blimp, it floats in middle water and tries not to touch anything... when young if you touch it it sticks like glue and extend for kilometres. The blimp end out filing the whole space, whatever the size. But it never touches anything. Getting older it gets rough and strong. But every bits that sticks somewhere dies..
I'm playing with this for a couple month. And it's possible to use new sprouts to stick it where you want it and leave it catch there. All you need is a very slick tool. Polished bamboo works best.