You should have a reason to run carbon, coral toxins from a densely packed tank, water clarity, etc. Otherwise it isn't necessary. If so, it should be in a mesh bag in a high flow area of the sump. A filter canister designed for carbon, or a reactor is a better way to go if available. Some folks don't run carbon (or at least some brands) because it can cause disease in Tangs. I run carbon because of a packed coral population, but I run a pellet version that I rise very thoroughly before use to reduce carbon dust. My Tangs have not shown any problem after several decades of use.