This is what works for me... Mag-Float 400 Magnetic Scraper for Glass, the ends are rounded over, not sharp, reducing the ability to scratch glass accidentally... No matter how careful you are if you get sand under the blade or the cleaning pad of ANY scraper you will scratch it, sapphire or not so take your time when nearing the sand bed and the silicone in corners.
Don't leave the blade in the salt water they will rust and are not meant to stay in the tank.