If you've ever opened a bag of Chemi-Pure you know it's more than just GAC. There's a few different types of very small resin pellets as well, some clear and some yellow. I'm no expert but I'm assuming these pellets are the much advertised ion exchangers. I used the product for years until I read Randy's article on the useless nature of exchange resins for sea water.
That said, I was always happy with the amazing water clarity that Chemi-Pure delivered. After I stopped using it I went through many different brands of carbon but never achieved equal results. Until I ran across the white paper for SeaChem Matrix Carbon. After trying it I was sold. Equal results to running Chemi-Pure without wasted money on useless exchange resins.