Tannins are substances present in wood. In the Aquariums, tannins most come from driftwood. Tannins give a brown tinge to the aquarium water. They also slightly decrease the pH which may affect the fish.
In some cases, Tannins may be desirable as they help create the appearance a genuine ecosystem. This may be necessary in biotope aquarium which may simulate an Amazonian river where the water in some areas has a brown color.
Tannins can be removed from the aquarium by means of water changes or through the use of activated carbon in the filter.
By properly treating driftwood (Boiling and soaking), tannins can be removed prior to adding driftwood to the tank.