Maybe it's a certain brand or something? I once had a 15G hex that was running for about 3 years with blue/purple gravel, and i never lost a paint chip, as far as i know.
Then, in the 46 gallon, I had black/red gravel for about 2 years, and did a gravel vac about once a month, with no paint ever falling off....
Perhaps it's certain colors or certain brands? very interesting.....
The question that this brings about is....what color of gravel is the "unpainted" kind? White or brownish tan?