Test your water for nitrate and phosphate, this will tell you if your skimmer is performing well. Phosphate should be zero and Nitrate should be low too, >10ppm. Also, are you going for a mixed reef or a specific species of coral? It really depends on what type of corals you want to focus on. Charcoal is widely used and it is a proven method for removing undesirable waste from the water. I would say that would be a good thing to add no matter what.