Randy Holmes farley---
What is meant by "Activated Carbon"?
It is a form of carbon with a very high surface area for absorbing organic molecules and catalyzing certain chemical reactions (like chlorine and chloramine breakdown).
I don't think that any organics are released by activated carbon unless they are being replaced by other organics. So the idea that things will eventually be released is not something to worry about. However, just like a kitchen sponge, it will eventually be "full", or more correctly, fully covered with organics. At that point, it is essentially useless.
So at some point, it needs to be replaced. How fast that is depends entirely on the flow over the carbon, and the level or organics in the water. I don't really have any opinion whether 5 days or 4 weeks is better. To some extent, I think it depends on how agressive you want to be in removing organics, and how much money you want to spend.
FWIW, I don't think you can really overdo carbon, within reson.
Carbon works because its surface is hydrophobic (like an oil). Organic molecules in the water that are wholly or partially hydrophobic will "want" to attach to this hydrophobic surface, and become stuck. Not all organics bind to carbon at all, but many will.
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Q) you mentioned before that you might try some testing of carbon effects on pH. Just curious.
OK, I finally remembered.
A) I took some tank water at about ph 8.26 and dumped in a large amount of marineland carbon. The pH rose substantially, to pH 8.6. I used more than 10% of the water volume in carbon. I expect that at more normal usage rates, the pH effect is much smaller, but likely measurable for heavy use.
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Interesting article-
http://www.resintech.com/pdfs/0501ph.pdf
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Actavated Carbon-
http://www.activated-carbon.com/solrec2.html
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http://www.hallman.org/filter/gac.html http://web.archive.org/web/200104201...ct/default.asp http://web.archive.org/web/200012050.../1/default.asp http://web.archive.org/web/200009180.../1/default.asp 
Using Carbon 
