I never used carbon on my 275 gallon system until a few months back when I went to Trop in Romulus and talked with Dick about it for an hour. I was converted and tried it. My water was immediately clearer (helping light reach my SPS and without any yellow tinge). Like crystal. Particulate matter lower too. I use black diamond and run a couple cups at a time through a generic media reactor fed by a maxijet in my remote sump. I run the stuff 24/7 and swap it out every three weeks of so (on the same schedule with my phosban). Running carbon has made a visible difference in my tank.
Just to be on the other side of the fence, I have never run carbon in the two years + that our 225 has been set up. No issues with water clarity or quality. Granted, my bio load is relatively light (2 tangs, 2 watanabeis, 2 bangais, a royal gramma and 2 orange anthias and a firefish goby). I do feed heavily most every day and run a skimmer. 10% water change every 2-4 weeks.
so it's basic, just clears the water in a visible manner, I don't know if I heard this right but it was while ago, Has carbon been linked to a certain disease that Tangs can contract? That's my main concern.
I also have tangs and run carbon for over 3 years with no problems, or diseases ever. However, i have also read that when you leave carbon in for more than 5 days it is pulled out all the organic matter it can hold, and will only remove beneficial trace elements after that point. This is even written on the label of my kent marine 'reef carbon'.
You can find successful tanks with everything from no use of GAC ever to 24/7 use. Generally people do find that its use benefits their tanks though, and I see no reason to run it intermittently. I'd run it 24/7 and have done so for years.
My carbon use thus far, is in the form of Pura Pads, placed within a 25 micron filtration bag that receives about 75% of the flow from my sump. I run two pads, and replace one monthly. The Concientious Aquarist book has a good read on the subject. It recommends having multiple bags, replaced one at a time on a set schedule. The article here http://www.bulkreefsupply.com/a3/Carbon-Directions/article_info.html has a good basic explanation of why to do it this way.
I only use carbon once a month for a few days, sometimes only every couple months. I just buy the premade floss/carbon for fresh water filters, rinse them and toss em in the overflow so the water can hit it but not cause any overflow issues. I've heard of alot of people not using it ever, just to clear up the water every now and again.
It's a strang feeling going from fresh water with carbon to salt water with carbon. Then to just stop using it feels almost scary.
also consider that certain corals, and other inverts when they die, or become threatened release things into the water, and the carbon does a good job of pulling it out. Carbons benefits far exceed that of water clarity. For those who don't run it, i would say try it for a month, and see if you can tell a difference. Ill give a good example. Mt PPE's stayed dormant for months, all ways looking splotchy, not two days after i started using carbon, the colors were solid and velvety again, and shortly there after exploded with growth. This is not to say this effects every coral, but i can literaly watch my ppe's and know when i need to change my carbon. there are others that have the same results.