Algae Scraper Replacement CartridgeThe scalper cartridge houses a 3 in 1 design. It easily attaches to your both sizes of seachems algae Scraper and retracts when not in use. The scraper is safe for salt and fresh water. The pad is the first choice when cleaning An acrylic tank, although the plastic can be used, Caution should be taken when working around gravel. The blade attachment easily removes tough algae but should not be used on acrylic tanks.