There is no harm from the tannins. Infact for some fisk like discus and
cardinal tetras there is some benifit. Many people filter with peat and
add blackwater extract because it softens the water some. Most people
with heavly planted tanks do not carbon because it will take out iron
and other nutrients that are used by the plants. The only time I would
use it is if I had to medicate the tank with copper for ich, etc.