i have an ancient magnet type scraper doohickey but for hard to get off green spot algae i use some old blue plasticy scrub pad thing they make for algae. a credit card will also work, and ive never scratched glass with a razor blade, but for those who do they make plastic razor blades, and holder for said plastic razor blade. the plastic blade doesnt work as well as a real one, and dulls quickly, but is safer.