Right on Gary its only 3/8". I'm fortunate to be bare bottom with this tank, but I've seen where people leave magnets in the tank and mare things up. The entire bottom is peppered with purple coralline and the sides are beginning to seed now. I am really uncomfortable taking a razor to glass since I've seen the magnets get miss aligned and scrape up a tank, I've been repeatedly told to use razors though. I normally scraped the glass with a credit card which will be impossible in this tank.