Adding natural driftwood to an aquarium often turns the water a distinct amber or tea-like brown. This change is caused by the slow, continuous release of organic compounds from the wood into the water column. These substances are known as tannins, and they are a natural component of many types of wood and other botanicals. Understanding the nature of tannins and their effects is the first step in deciding whether to embrace or manage this natural phenomenon.
Defining Tannins: The Chemistry Behind the Color
Tannins are a class of polyphenolic biomolecules widely distributed in the plant kingdom, acting primarily as a natural defense mechanism. They are large, complex compounds that plants produce to deter herbivores and protect against pathogens and decay. Within the structure of driftwood, these compounds are stored within the wood’s cellular tissues. When the wood is submerged, water begins to penetrate these tissues, dissolving the stored tannins and causing them to leach out.
This leaching process gives the aquarium water its characteristic color, which can range from a pale yellow tint to a deep, dark brown. The specific hue and concentration depend on the type of wood, with some varieties, like Malaysian or Mopani driftwood, releasing particularly high amounts. The compounds released are often referred to as tannic acid. While they are a form of organic acid, they are generally harmless to aquatic life at the levels typically found in a home aquarium.
Impact on Water Chemistry and Environment
The presence of tannins creates what is often termed a “blackwater” environment, mimicking the natural conditions found in many tropical river systems, such as the Amazon basin. The most immediate chemical effect is a slight reduction in the water’s pH level, making it more acidic. Tannins act as a weak organic acid, but the overall change is often moderated by the water’s existing carbonate hardness (kH), which acts as a buffer.
In addition to lowering the pH, tannins contribute to a mild softening of the water by binding with some dissolved minerals. The dissolved organic compounds also possess mild antiseptic and antibacterial properties. This can help inhibit the growth of certain harmful bacteria and fungi, contributing to the overall health and reduced stress of the aquarium inhabitants.
Many popular aquarium fish, including various tetras, dwarf cichlids, and angelfish, thrive in these blackwater conditions, which can encourage more natural behavior and even spawning. The tinted water reduces light penetration, creating a dimmer environment that is preferred by some light-sensitive species. Tannins help create a more biologically appropriate environment for these specific aquatic animals.
Managing Tannin Leaching in Aquariums
For aquarists who prefer clear water, there are effective strategies to control and minimize the release of tannins from driftwood. The most proactive step involves preparation before the wood is placed into the aquarium. Pre-soaking the driftwood in a separate container for several days or weeks allows the bulk of the initial compounds to exit the wood. The water should be changed whenever it becomes noticeably dark until the leaching slows significantly.
Boiling the driftwood is an accelerated method that rapidly extracts and removes a large quantity of tannins from the outer layers of the wood. The heat increases the rate at which the compounds dissolve, and this process also has the secondary benefit of sterilizing the wood.
For ongoing control after the wood is submerged, chemical filtration media can be used within the aquarium filter. Activated carbon is highly effective at adsorbing and removing the organic molecules responsible for the coloration and should be replaced regularly as it becomes saturated. Specialized synthetic resins are also available; they efficiently remove tannins and are often rechargeable for repeated use.