Here is a acticle from another website i found.__________________________________________________________________________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.
How Much Carbon Should Be Used?
More is not always best, and using the least amount of carbon necessary is recommended. The general concensus here is that a quantity of about 3 level tablespoons of GAC per 50 gallons of actual tank water volume should be sufficient to keep a saltwater aquarium or reef tank system color free.
How Often Should Carbon Be Changed?
Like most other absorbing compounds, carbon can only absorb as much as it can hold. Once it has taken in as much as it can, it becomes exhausted, which means it is unable to absorb anything more. For this reason it needs to be changed and replaced, or rejuvinated for reuse. Not all aquariums run the same, so as far as determining when to change the carbon, this is something you will have to figure out on your own. By sight you definately know it's time when the aquarium water begins to get a yellow tinge to it.The bottom line is that it's best to use smaller amounts changed more often. ranular Activated Carbon FAQ provides a WARNING to take into consideration; not changing all the carbon too quickly if your water is significantly yellow, as this can expose corals to too much UV light abruptly which can lead to coral bleaching, and possible death
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I use very little carbon one weekend a month.If you have a good protein skimmer and do frequent water changes you shouldn't really have to use Carbon. I have a 14oz carton and my tank has been running over 4 months and i have over half of the carbon left.
Hope that helps.
