Greeting,
This is my first post and I hope it is a welcome one.
This subject is debated endlessly by tropical fish hobbyist. In my 40 +years of raising tropicals, mainly live bearers, I have come to the conclusion that except for removing medications after treatment, it is more trouble than it's worth. Since I've stopped using it I have had far fewer problems with my fish. I currently have 200 large corner filters running and you won't find any carbon on the premises. I don't even use it after treating fish. I prefer a large water change with aged water.
I'm providing the following information, which I hope will help. Remember, this is MY experience on the subject. Your mileage may vary!
Granular Activated Carbon
* Activated carbon effectively removes many chemicals and gases, and in some cases it can be effective against microorganisms. However, generally it will not affect total dissolved solids, hardness, or heavy metals.
* Any granular activated carbon filter has three inherent problems.
--First, it can provide a base for the growth of bacteria. When the carbon is fresh, virtually all organic impurities (not organic chemicals) and even some bacteria are removed. Accumulated impurities, though, can become food for bacteria, enabling them to multiply within the filter.
--Second, chemical recontamination of granular activated carbon filters can occur in a similar way. If the filter is used beyond the point at which it becomes saturated with the impurities it has absorbed, the trapped impurities can release from the surface and re-contaminate the water, with even higher concentrations of impurities than in the untreated water. This saturation point is impossible to predict - or detect.
--Third, granular carbon filters are susceptible to channeling. Because the carbon grains are held (relatively) loosely in a filter, open paths can result from the buildup of impurities in the filter. In this situation, contact time between the carbon and the water is reduced, and filtration is less effective.
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Hope this helps.
Frank