Ken Feldman's articles that Pat linked to are great, but they get pretty technical. When Ken does his testing, he backs it with all kinds of scientific info which is awesome, but can make your head spin.
The best carbons are:
1. Bulk Reef Supply ROX 0.8, (Extruded peat .8 mm. Looks like broken mechanical penicl lead, HCL acid washed).
2. TwoLittleFishies Hydrocarbon or BulkReefSupply Lignite, (HCl acid washed).
3. thefilterguys Bitum for Kidney dialysis, (Nitric acid washed, dust free and pH neutral).
4. Matrix Spherical Bitum.
5. Elos very small pellet Bitum.
ROX 0.8 dosage is 1/3 cup/50g, all others are 1/2 cup/50g. They should all be changed every month.