Denitrification catalyst 5pcs for Maxspect Anaerobic BlockFor optimum efficiency, it is recommended to change the denitrification catalyst every 2 months and the Nano-Tech Anaerobic-Block should be cleaned or replaced every 6 months. However when cleaning or replacing them, DO NOT change them all at once. Clean or replace only half so that enough bacterial colonies remain to grow in the freshly cleaned or replaced Anaerobic-Block.