I use "BLACK DIAMOND CARBON by MARINELAND"
Its very good but its larger size than some others.
Bellow is it's description
"Eliminates odors, discolorations & harmful organic substances better than any other brand. Heat activated bituminous coal based granules are specially sized and far more efficient than coconut, wood, or peat based carbons. With black diamond you get more carbon. As much as 60% more of other prewashed carbon weight is water not carbon black diamond contains less than 2% moisture. more carbon, better performance better value, and phosphate free.!!!"