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Hi guys. I've learnt that you should change your biochemical sponge frequently and my question is why? Yes, I know how recycling works: NH₃ -> NO₂ -> NO₃. And I also know that the filter is there to cultivate the beneficial bacteria - Nitrosomonas, Nitrosospira, and Nitrospira. I just need to know why we should change the foam once a month (this, of course depends on the fish-gallons ratio). Are those biochemical foams coated with a layer of germs that die out over time or something? I really hope someone would explain this to me. Thank you.