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Are Wood Tannins bad for Fish? And Reasons for That

Are Wood Tannins bad for Fish? And Reasons for That

When attempting a more natural aesthetic appeal for your aquarium, or trying out different aquascaping methods, some Driftwood, leaves cones and other botanical elements can make an essential addition.

Though these botanicals may look exquisite, they can affect the water in your aquarium over time. You may have noticed a yellow to brown discoloration in your clear tank water, and this is ideally because of the “Tannins” present in the natural elements in your tank.

What are Wood Tannins?

More specifically, most aquarists invest in driftwood and other wood forms to decorate their tank, thus we are focusing on wood tannins specifically.

Tannins, known as Tannic acid are a natural substance that you will find on wood, bark, rhizomes, roots, and even some fruits. They form part of the polyphenol family, an antioxidant substance that helps to preserve tissues against cellular aging. Though Tannins are a natural part of the decaying process essentially.

Tannins are acidic, and they can slightly alter the color and acidity levels of your Aquarium water, though many aquatic animals can get used to them and eventually even benefit from them.

Therefore a little Tannic acid buildup in your aquarium may not be all that bad, so let’s look a bit further into how they affect Aquatic life.

Are Wood Tannins Bad for Fish?

To answer our question at present, no, tannins are not essentially bad for your fish, however, they can alter the water parameters which in turn may either benefit your tank inhabitants or not so much.

Before introducing Driftwood and other woods to your tank, it is crucial to understand their effect on your aquarium and your benefits.

Did you know that some fish species such as Plecos and Catfish species require Driftwood as a form of nutrition to aid in digestion, much like fiber and probiotics for humans?

One of the main issues with tannins is that they change the water pH levels of your tank, causing it to become more acidic. If this process happens too quickly it could negatively affect some fish species, especially shrimp that are sensitive to fluctuations.

On the other hand, you need to introduce a substantial size of wood to your aquarium for the fluctuations to be so critical. If you have a water hardness in your tank of more than 5KH, it can essentially neutralize the acidity caused by tannins, and soften the water slightly.

Unfortunately, not all aquatic life thrives on soft and low-pH water, so it is important to consider the species of fish and invertebrates that you are keeping, or planning to keep.

Pros and Cons of Tannins

The pros of Tannins By far Outweigh the Cons in the right circumstances;

Benefits of tannins

  • They are antioxidants with antimicrobial and antibacterial properties.
  • Tannins in Indian Almond Leaves improve the breeding rates of fish and the survival of the females.
  • It contains trace elements that induce the metabolism and molting of shrimp.
  • The Discoloration caused by tannins can reduce the development of algae, as it darkens the tank.
  • Broken down leaves, and wood, or as it is called detritus, form food for shrimp and fry, and the microorganisms and biofilm on wood use good nutrition for bottom-feeding fish.
  • As an antioxidant Tannins are excellent to improve immunity in your fish.

Some Disadvantages

  • The yellow or brown discoloration is not aesthetically pleasing and can cause your tank water to look dirty.
  • Tannins’ lower pH levels and water hardness, thus it will affect your water parameters.
  • There may be an increase in TDS (Total Dissolved Solids) in your tank, which can cause stress to shrimp especially.

Reducing Tanning Leakage in Your Aquarium

If you prefer to reduce tannin levels in your tank and feel that they are negatively affecting your fish or shrimp you can follow these steps to reduce tannin leakage, though it may not stop the process entirely.

  1. Use activated carbon in your filter and replace it regularly.
  2. You can use an organic filtration resin, “Seachem’s Purigen” which has been said to be the best by many aquarists.
  3. Try to cure your driftwood by submerging it in dechlorinated water for a week or two.
  4. Regular water changes, which form part of your tank maintenance regime should also keep the water clearer.
  5. You can always opt for faux driftwood if it is only used for aesthetic purposes.

Final Thoughts

In the end, Tannins from Driftwood, and other botanical sources essentially do more good than harm to most fish and aquatic species. However, tannins will affect the water’s pH and hardness, causing the water to be slightly more acidic and softer. With most fish species this is not much of a problem, however, some species, especially shrimp that are sensitive to fluctuations, or soft, acidic water, will not reap the full benefits. Lastly, tannins will cause discoloration in your tank water, which is not all that appealing.

Frequently Asked Questions

Can Tannins Kill Fish?

Tannins will not kill fish, however, some species of fish can become stressed and ill over time with the changes in acidity of the water, if not kept under proper control.

Are Tannins Good for Aquarium Plants?

Tannins can increase acidity and soften the water in your tank, which is beneficial for some plants. They similarly contain antioxidants which can be good for plants. Though, it must be said that the brown discoloration of the tank water may block out essential light needed for plants to grow.

Which Fish Species Prefer Tannins?

Most fish species that originate from tannin-rich water such as the Amazon, Congo, and Orinoco, where the water is slightly acidic and soft, will thrive in tannin-rich water. Some “Blackwater” species include;

  • Betta Fish.
  • Gourami’s.


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