Just out of curiosity, what exactly is a skimmer? i have seen a million ads for them but never bothered to ask what they are used for.
It is used in saltwater setups to remove dissolved organic compounds. Since you posted in the Freshwater forum, I will also say you can get SURFACE skimmers for freshwater that will remove a protein layer from your water if you have one. Freshwater surface skimmers also increase oxygen levels because of their design.
note: saltwater skimmers work by making millions of tiny bubbles, but that process doesnt work in fresh water.