I have been using Seachem Purigen in the filter section of my Fluval Spec V aquarium for a few months now.  I recently pulled up the filter material and the Purigen has now turned a nice, dark brown patina.  Their instructions have the following to say about when to clean: “Exhaustion is indicated by a pronounced discoloration of the beads to dark brown or black.”  While not yet black, I couldn’t help myself

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