Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Activated Carbon has been addressed, too. It has great affinity for larger molecules, especially organic (strict definition) molecules.
It has less affinity for the simple molecules that are most of our fertilizers.
It can remove chelated minerals, but does not remove much, if any of things like KNO3, KH2PO4 or K2SO4.
It can remove most dyes (such as dye based medicines) and most other medicines.
It can remove tannins, but the wood or other source of tannins will keep on producing more tannins, so I find it better to do water changes (which the tank needs anyway) to keep the tannin level lower.
As suggested above: If you cannot tolerate any tannins, then do not place organic materials (wood, peat moss, dried leaves) in the tank.
If a little bit of tannins is OK, you can help the material get rid of a certain amount by boiling or pre-soaking, but you will not get rid of all the tannins that way, and some stuff does not stand up to pre-treatment very well. Better just to use a few leaves, for example, at a time.