While tannic acid might sound like some sort of scary compound that you don't want in your tank, it usually only leads to a slight pH change and some funky water color. You'll probably encounter tannic acid only if you add a certain wooden decoration to your tank.
Although certain aquarium chemicals contain tannic acid, most fish hobbyists encounter the compound from adding driftwood into their tanks. When placed in water, the driftwood begins releasing tannins into that water. FishChannel.com notes that Malaysian and African driftwood release the most tannins, while American driftwoods leach fewer tannins.
Once driftwood leaches tannins into your aquarium, your water color likely will take on a yellowish-brown appearance. It often looks like weak tea. While water changes will change your water somewhat back to its normal color, the change is only temporary, as the driftwood will continue to release tannins. Over time, the amount of tannnis released diminishes and your water will return gradually to its nice, normal clear color.
Water pH and Hardness
Plop a piece of driftwood in your tank and test your pH a few days later. You may notice a slight drop in the alkinity and hardness of your water. That's because tannic acid reduces water pH and hardness. Depending on your water chemistry and the type of driftwood you add, the change can be significant, but it typically is not. Fish who prefer alkaline conditions will become stressed if the tannins cause your water pH to swing into acidic territory, while fish who enjoy acidic water -- the majority of tropical fish -- likely won't give a hoot unless the change is dramatic.
Soaking and Boiling
If you'd rather your water not look like a giant container of tea or you don't want your pH to drop, boil and soak your driftwood until most of the tannins leach out. Boiling sterilizes and hastens the process of the wood releasing its tannins. However, because the process often takes weeks to see any noticeable change in the amount of tannins released, it's more feasible to soak the wood in a container of dechlorinated water. Soaking also can help the wood sink in your tank if it doesn't already. Weigh the driftwood down so that it's completely submersed in the container, and change out the water when it darkens. When the water remains fairly clear for two or three days in a row, you can place the driftwood into your tank without worrying about quick and excessive discoloration. Note that it can take months before the wood will stop leaching tannins completely.
If you've already placed the driftwood into your tank and the tannins are causing you grief, add activated carbon to your filter and perform weekly water changes of 20 percent. The combination of regular water changes and activated carbon will remove most tannins while they leach from the driftwood.
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