10-14-2008, 11:36 AM
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I don't use carbon on any of my tanks. Sure, the conjecture relating to hole-in-the-head is simply that, but I still don't see that the risks outweigh the rewards.
Fresh activated carbon is *very* good at removing organics from your water. This means that any good organics in your water (i.e. compounds that could be used by your fish or plants for healthy growth) aren't free for use. Water changes help to replace some of these materials, but again if you have a high flow rate through a filter with fresh carbon, those added-back materials aren't going to last very long. If you're using liquid fertilizers, using activated carbon would be like flooring the gas and brake pedals at the same time. I also have a bit of blackwater going on in some of my tanks that would be impossible to achieve while using carbon.
So if my tanks are healthy and beautiful and my fish are happy and breeding, why would I spend the extra money on constantly replacing the carbon? Provided you're doing regular water changes (which are much less of a pain both cost and time wise for freshwater tanks than they are for saltwater) then the two benefits Pasfur talked about - nitrate buildup and hardness depletion - aren't real concerns. In fact, many freshwater fish (many new world cichlids, the tetras, barbs, loaches, etc) prefer softer water and would actually benefit from this buffer depletion effect, provided pH swings were prevented. pH itself is pretty easy to control in a freshwater tank since pH-raising additives like calcium aren't being dosed and tapwater pH tends to be fairly steady.
I guess I'm sort of rambling at this point. I recognize completely why these concerns would apply to a saltwater tank but I believe wholeheartedly that the use of activated carbon in freshwater systems is just a gimmick the industry has come up with in order to have hobbyists send them parts of their paycheck on a regular basis.