When using razor blades to scrape algae from the walls of your aquarium, make sure to clean them thoroughly before use. Most razor blades come packaged with an oily coating, which prevents them from rusting. While this coating is minimal and may quickly be removed via filtration, it is just another contaminate you don’t need in the tank. The oil could potentially harm invertebrates, especially if it has any metal flakes from the manufacturing process in it. To remove, wipe away as much as you can with paper towels and soak the blade in rubbing alcohol and then RO water. Be sure to remove any moisture though, as it can cause rust.