I think most people will agree that carbon is a good idea.
Previously I had it in a bag laying in the sump (passive), since I got a phosoban 150 reactor and run Black Diamond, it really cleared up the water. I will get better use out of the carbon through the fluidized reactor, and more bang for the buck!
Carbon = yes
Phosoban reactor makes it cost effective over the long run.
GAC is very good to help with water clarity, takes the yellow out and helps with any chemical warfare between corals.
I did a lot of reading and found that "a bag of carbon" just sitting in a sump is not going to do you any good. Water needs to be forced through the carbon, as reef_doug explained. I have a piece of capped off PVC with a hole in each end. I use an MJ power head to push water through the carbon. When I get my tank back up and running I plan to use some of the water from the sump return so that I do not need the extra power head in the tank.
i made a carbon reactor out of some 2 " pvc and a maxijet 600?? i dont remember its a smaller one... was easy and you for sure see a diffrence when it runs but need to change it every 2 weeks.. gets costly if you want to run the high end carbon wich is worth every penny
I dont believe you "NEED" to have water forced thru carbon for it to absorb impurities in the water. However, I do believe that it makes for better more efficient absorbtion by the carbon. Carbon can only absorb chemicals that come with in a given proximity to the carbon for the ions to transfer. Trapped in a filter bag slows the rate that water can pass thru it. If it is sitting in your sump the amount of water traveling thru the bag is greatly reduced compared to actualy having the water forced thru the media.
That just the way I see it. I have done a good bit of reading on the subject of carbon and based on what I have read I have concluded its better if its forced.
I have carbon running 24/7 and all my water is forced thru it.
I actually have one of those small in sump canister filters. I replace the carbon roughly every month, I also sprinkle in some Rowaphos phosphate remover. Basically you can't lose using some carbon, it removes impurities and helps clean yellow water. It really can't hurt anything when you use a reasonable amount and change it at regular intervals.