What I remember is you soak it in bleach to "reactivate " the purigen after it has been used a while (then dechlorinate). I would use a final fine filter pad after the purigen to capture any particles in a cannister filter before water returns to the tank. The pads are for particles, the purigen (or carbon) is for chemical filtration. It's all good but don't learn to rely on the chemical adsorbers, water changes are best.