Jon, I use carbon about 1 week per month. If I miss a month, I still do not get any yellowing of the water. I have gone 4 months before and did not see any coloring at all. I check this in a white bucket during water changes, where yellowing is often most apparent.
To note, I do not have any soft corals, and I believe they contribute more to this that SPS.
To answer your question though, I would use carbon to get full strength light in my tank.
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Brad
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