Mine 3', 2', and 2' do not appear to scratch acrylic, neither do irwini, nor Pterodoras granulosus. Even if they did, it would hardly be noticeable as it'd be on the water side and the refractive index of acrylic and water is not that different to strongly scatter light.
Air and acrylic have vastly different refractive indices so any scratch is quite visible.
The only fish from the top of my head that scratches the acrylic badly enough is a rasping pleco.
What pics are you referring to?