I personally don't like using carbon cause if you had ferts and what not it will take them out of the water. I would use a mechanical media (coarse sponge) at the bottom where the water comes in then a biological media which would be a finer sponge then another biological media but this time some sort of ceramic tubes as the third media. Then if you ever need to dose medication for sick fish treat them and once the dosing of medication is done and the sickness is gone you could replace one of the biological medias with a carbon media to get the medication out of the water.