The reference is probably to active carbon. If that is so then it serves two purposes as part of the substrate mix:
- In the first few weeks the carbon is still active and it will absorb any (hopefully all) substances that could enter the water column - tannins, pigments, organic molecules all of which can come from peat.
- After that the carbon becomes inactive but because of its big surface area it can serve as and extra space for bacterial growth. Bacteria is of course a natural and important part of the exchange of nutrients between the plant roots and the substrate.