09-03-2010, 10:39 PM
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How long carbon lasts in a filter is more determined by the amount of waste that is present, the grade of carbon being used, and the amount of carbon being used. Carbon has caught a bad rap since the internet was invented. There are a lot of misconceptions about its use, its benefits, or any drawbacks.
On the use of carbon I guess we will have to agree to disagree, but I have seen first hand the many benefits it has to offer when used properly.
One thing to keep in mind when using a canister filter is organic waste. While the filter promotes the growth of aerobic bacteria, it does not offer the benefits for anaerobic.. which ultimately break down waste beyond the level of nitrates. Canister filters that are not cleaned and maintained regularly will contribute to what is known as "old tank syndrome" even if water changes are done often. Solid waste build up in the canister will break down over time, which leaves the nitrate level that the aerobic bacteria don't handle. This is then circulated back through the tank. Carbon absorbs excess nitrates, (among other things) and as long as it is changed often enough for the amount of waste in the tank, it can be a huge benefit to any aquarium.
I'm not sure where you heard (or how you figured out) that carbon is only effective to 8 days... but that is already scientifically tested... again, it is dependent on the grade of carbon, amount of carbon, and amount of waste. Is it possible to use up carbon in 8 days... sure.... if you use a very small amount of a cheap grade, such as pelletized, and have a large amount of waste to deal with. It is more about using it properly to get the desired results.