No fish need carbon. If you do enough water changes you should not need carbon at all. There are some negative effects of using carbon, the theory is that it removes vital micronutrients and trace elements.
Seachem Matrix has less surface area total, but more bioavailable surface area. The difference is that with Fluval and Eheim's biomedias the pores and channels are so small that when a biofilm of beneficial bacteria start growing it actually blocks off those pores and channels, reducing the available surface area. The pores and channels in Matrix are large enough to allow a biofilm to grow without getting blocked up. The Matrix is also supposed to allow for denitrification. Whether that is true is arguable, some say yes, some say no, some say the people saying yes are really just seeing a difference after cleaning the debris out of the filter (which will also lower the nitrate concentration in the water). I use and recommend Matrix now. It is at least as good as the other biomedias out there, and with the possibility of denitrification it is worth using as the only biomedia.
