04-02-2011, 12:42 PM
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Carbon removes "stuff" from the water by adsorption. Some of the "stuff" removed will be nutrients that the plants can use. In planted tanks, carbon should not be employed on a regular basis but only when needed such as to remove a medication after treatment. Not only does it remove nutrients, but the plants themselves act as detoxifiers of much of the "stuff" carbon targets, so it is less expensive (because carbon needs replacing regularly) to let the plants do the work for you.
AQs noted, the carbon will exhaust its adsorbtion capability; the time for this depends upon how much carbon, and how much "stuff" it is adsorbing, which in part depends upon how many fish in the tank, what's in the water, fish foods added, etc. My point in mentioning this is that eventually the carbon will cease to work and at that point is no issue as it is simply "filling space." But until then, it will remove nutrients to some degree. If you can do it easily, I would remove the carbon.
There is also the issue that there may be other chemical media in with carbon, and this could have further impact on plants.