carbon usually only lasts for about a week. it really isn't necessary in reef tanks and lower quality ones may even release phosphates into your water. What I would recommend is a higher quality organic waste remover, something that mimics what carbon does but is much more effective and safe. Some options would be chemi-pure or chemi-pure elite (the latter of the two has extra media in it to remove phosphates), purigen by seachem, biochem zorb, or any other products similar to these. The will last a lot longer than the carbon and some of them are even rechargable making them last even longer. Personally I love chemi-pure elite, but many people have arguments for what they use to.