Carbon ias fine to run it does remove some trace elements.....not calcium but others but algae and other things will remove more than carbon. if keeping corals though you can always supplement these. On a side note what you want to look at is what the carbon is made from (wood, lignite coal, etc....) IMO the best is the lignite coal but you also want to consider that carbon can not just take things from the water but also release things like phosphate. What i think is best is to get a phosphate test kit fill with RO water and put drops of test solution in tube then drop in a piece of carbon. If you see color streaks coming out of carbon then it is putting phosphates in the water. this is why i am hesitant to buy cheap carbon.
Also carbon needs to be placed in an area where water is forced through it and not able to just go around it. Besides just binding certain pollutants in the water to it another benefit is that it is also porous and acts as miniature LR where bacteria can colonize it (may be minute and pointless when considering size of tank and LR etc... but every bit helps....can't hurt). I like to run carbon regularly. IMO using for a couple days here and there does nothing but help remove yellowing compounds from the water which could possibly be detrimental to corals because more light is let through then..... but if thats what you want to use it for then fine but as far as a source of chemical filtration you should use more regularly.
i'm sure you weren't looking for an in depth rant like that but carbon is one of those things that comes with multiple different opinions on how, when, why, if ever, it should be used so i elaborated. hope it helps.
