I did this mod too on my scrubber in an old 2 gallon deco a couple years ago because the stock mag float required too much "oomph." Since the tank was topless and fully filled, water would always spill when I used it.
I used the scrubby sponge on the inside and while it didn't "scratch" in the traditional sense, it definately "hazed" the tank. I think the scratches were so fine that is showed up as a haze.
So just be careful with non-glass surfaces - over time this may be a bit too abrasive. I can take a pic of the old one in a couple days so you can see the haze - Im in the process of setting it up to house frags Im getting rid of.
BTW - anyone local to Philly can have a free kenya frag. Or I will trade 4 decent kenya frags for an acan. (fat chance, I know, but dollars-wise, it's probably a nice trade).