Some aquarium fish come from waters that look more like cola than tap water. This type of water contains high levels of tannins, an organic compound that discolors and acidifies water. In the aquarium hobby, such water is called blackwater -- not to be confused with the similar medical, sewage or paramilitary terms. While the fish from these regions thrive in such water, other species of fish fare poorly in blackwater conditions. Additionally, some aquarium hobbyists dislike the look of stained water. Fortunately, it is very easy to prevent and reverse this situation.
The Downsides to Tannins
Tannins don't just discolor the water. They also tend to bind up minerals in the water, softening it and lowering the pH, hardness and alkalinity. All aquarium fish have ideal ranges in terms of pH. While fish found in blackwater conditions thrive under these acidic conditions, these conditions can stress fish that have adapted to different water parameters. For example, livebearers from Central America and the cichlids of the African Great Lakes need hard, alkaline water to thrive and will get stressed and suffer from poor health in blackwater conditions.
An Ounce of Prevention
With a little forethought, you can prevent tannins from entering your aquarium in the first place. Certain aquarium decorations leach tannins. Hobbyist that keep blackwater fish may use these decorations deliberately to create these conditions, though if you want to avoid tannins, it is as simple as not using these decorations. If you don't want tannins, avoid Malaysian driftwood and African moponi wood. Also, do not use Indian almond leaves, oak leaves or peat as your substrate, since these decorations all leach tannins into the water. Decorations are the main way tannins get into aquarium water accidentally.
Filtering It Out
You can use activated carbon to bind up tannins in aquarium water. Activated carbon consists of charcoal granules, heat- and chemically treated to make them porous. This increases the surface area and makes them more reactive and better able to absorb tannins and other compounds. Any aquarium filter that contains activated carbon can typically clear an aquarium of tannins in several hours. If you have a steady source of tannins, you may need to replace your filter inserts if the filter stops clarifying the water.
When Blackwater Is a Good Thing
Always make sure you want to prevent or remove tannins before you do so. Many aquarium fish originate in tannin-rich waters and will show their best coloration and thrive in such conditions. Fish like cichlids, tetras and catfish from the Amazon and Congo Rivers thrive in these conditions. Some species even respond to tannins as a breeding trigger, and will spawn in the presence of tannins. You should not remove tannins if you keep such fish.