I need to start by making an embarrassing confession that has lead me to this question. I recently tore down a 3 year old 10g, and set up a new 29g tank. Both aquariums used HOB power filters, with replaceable filter cartridges. The manufacturer suggests changing the filter cartridge every 2-4 weeks. However, in the entire 3 years that my 10g was set up, I never ONCE changed the cartridge (Yikeees! :;smack horrible fish-keeper, horrible fish-keeper....). Years ago when I set up my 10g tank, I followed advise to "never change the filter media" and mistakenly took this to mean the filter cartridge. Every few water changes I would removed the bio-fiber and the cartridge and rinse them in tank water, but always plopped them both back inside the filter box. I NEVER replaced the cartridge. I only recently realized my mistake when I purchased my new 29g and started reading about the nitrogen cycle again. The weird thing, is that even with the really old (and ineffective cartridge, I assume?), my tank seemed pretty healthy. I did 10% weekly water changes, and water tests always showed no ammonia/nitrite. My plants grew well, and I never had any strange fish deaths. I guess I am struggling to understand the REASON this cartridge must be replaced so often. -What exactly do those little pieces of carbon inside the cartridge do for my tank? The one in my 10g was obviously doing nothing, but was it harming my fish? -Are there differing opinions about how often the cartridges really need to be changed? -Did I just get VERY lucky?