Well you learn something new everyday - your post made me curious since I had not heard any negatives on carbon - I found this -I stole from another site(not other message board) - link provided on request,no author listed.
"How Often Should Carbon be Used?
The two main concerns about using carbon in a saltwater system is that carbon often leaches phosphate into the aquarium, and that it removes necessary trace elements needed by reef animals, particularly corals.
The Phosphate Leaching Issue
As far as phosphate goes, you want to remove this element from your tank, not add it. A high phosphate accumulation in saltwater aquariums can lead to aggressive hair algae blooms that are difficult to get rid of, and since some brands of carbon may leach phosphate into an aquarium, you should test the carbon you are using, or going to use, for leaching. If you find any relevent traces, change to a different brand.
The Trace Element Absortion Issue
There has been an ongoing controversy amongst aquarist for years if you should use carbon continuously, just when needed, or even at all. The trend leans towards using carbon only when there is really a reason its use is warranted, because a healthy system, particulary one that has an adequate protein skimmer installed should not require it. Richard Harker's iodide absorption test results lead him to conclude that, "the use of carbon has minimal impact on iodide concentrations and that passive use of carbon has no lesser affect on iodide levels than active use." Our conclusion here is that if you decide to use GAC on a continuous basis and are concerned about the possible loss of trace elements, add some trace element supplements in conjunction with the activated carbon, as well as bypass the water flow through the carbon for a short period of time before and after supplements have been added to allow it to be absorbed by the reef life and not the carbon."
Both scenarios make a lot of sense to me (especially the phosphate) I was having a terrible time with cyano outbreak - after reading that and realizing I changed my carbon and the cyano started, I would be willing to bet the brand carbon I was using was leaching phosphates. Anyway thats the info I found, hope to help.
