If you have driftwood in your aquarium, you probably know that it leaches tannins into your aquarium water over time. Tannins are known to change the color of water and also lower its pH, making it slightly more acidic.

So, with these effects, are tannins safe for fish?

Tannins don’t kill fish but can cause a slightly acidic environment over time. They also cause aquarium water discoloration and diminish the aesthetic appeal of your tank. The increasing acidity can also stress and harm fish over time.

Excess tannins inside the aquarium can kill fish that prefer alkaline water. Some fish species are even triggered to start breeding if there is enough tannin in their environment.

Driftwood is a major source of tannins in aquariums
Driftwood leaches tannins in fish tanks.

Are tannins good for all fish?

Tannins are not good for all fish because some species are sensitive to changes in water pH. Different species of aquarium fish have different ideal pH ranges for survival.

Therefore, tannins may be good for fish that prefer slightly acidic conditions. Otherwise, the fish will suffer stress and even death in extreme cases.

For instance, the Central American livebearers and the cichlids from the African Great Lakes require hard, alkaline aquarium water to live and thrive. Exposing the fish to lower pH makes can kill the fish overnight.

Will driftwood tannins kill fish?

Driftwood tannins will not kill fish since most aquarium fish originated from neutral to slightly acidic water bodies. 

Still, a high acidic level of the aquarium water, not easily contributed by driftwood tannins alone, can threaten the life of some fish species. An increase in water acidity causes the thickening of the mucus on fish’s gills.

As a result, there is a restriction on oxygen levels that can pass through the clogged gills. Changes in water to unfavorable pH levels also damage the skin and eyes. 

Benefits of tannins in an aquarium

Here are the benefits of tannins in an aquarium:

1. Tannins lower water pH

For aquarium hobbyists who are keeping fish that like acidic waters, you can use driftwood to lower the pH of your water and make it favorable for your fish.

Fish species that thrive in acidic environments are the tetra, guppy, and betta fish. These are hardy fish, but you still want to monitor the pH of your aquarium to ensure it stays stable.

While you can keep some fish in hard tap water for a short period, ensure you soften it and correct the pH as soon as possible.

2. Tannins reduce harmful metals in water

Water contains different quantities and types of harmful metals that can poison and kill all your fish. Examples of metals that are dangerous to fish include mercury, lead, and sodium.

The recommended metal concentration in aquariums is below 5g/cm3.

Adding tannins in the aquarium water reduces the concentration levels of the metals to deliver a safe environment for your fish.

A research publication by V. U. Kavitha on Springer states that “Tannins being a class of polyphenols easily extracted from natural resources shows an excellent affinity towards the metal ions in aqueous solutions that can use as a material of biosorbents…” (Source)

3. Tannins improve aquarium biodiversity

Blackwater brought about by tannins contains a lot of organic and decomposing matter, which helps microorganisms thrive and break down toxic gases.

Including more organic matter and material, like driftwood and almond leaves, inside the water yields an increasing release of more tannins.

Fish like plecos rub against driftweed and can help release more tannins in your tank.

4. Assists in reducing diseases

Studies have shown that tannins have an antifungal effect and can be beneficial for aquariums that are prone to fungi. To some extent, adding driftwood that releases tannins that can help you control fungal diseases in your fish.

How to keep tannins in an aquarium

If you keep fish that can benefit from tannings, you can constantly add them into your tank in two ways:

  • Adding driftwood as decoration in your tank.
  • Adding organic tea that contains tannins into the tank.

You can also add beech, Indian almond leaves, and pods from monkey pot and magnolia into the aquarium to increase tannin levels.

See more in the video below:

Too many tannins in the aquarium

Too many tannins in the aquarium can lead to the build-up of acidity and the delivery of tannins, causing water discoloration

Various fish species, including catfishes and tetras, love the blackwater environment that’s brought about by tannins.

Lowering the water pH can ultimately harm certain fish species like the African cichlids and Sailfin Mollies that love alkaline waters.

Managing the effect of tannins

Tannins in tanks designed for beauty must be managed to reduce the color and pH change effect on water.

Here’s how you can control these effects in your aquarium:

1. Perform water changes regularly

Regular water change can keep the tanning concentration at low, healthy levels in your aquarium.

An aquarium with mild color requires a 50-60% water change, while an aquarium with a heavy color change requires a 75-80% water change.

Replace the aquarium water after two or three days. For the heavily affected tank and mildly affected tank, water replacement should be done for more than ten weeks and at least six weeks, respectively.

One main disadvantage of this method of tannin removal from aquarium water is that complete water cleaning takes longer. In addition, fish not used to regular water changes and unstable water conditions can be stressed and harmed.

2. Use activated carbon

Activated carbon (activated charcoal) is a popular filtration method for aquariums. The carbon has a large surface area for absorbing contaminants in the aquarium.

Black or bituminous coal is the most common carbon source used for making activated charcoal. Granular activated carbon, made using raw materials with a high carbon composition, such as coconut shells and peat, is the type of activated charcoal used mainly for treating aquariums.

Activated carbon can effectively filter chemicals like tannins and chlorine in the tank. It can also help eliminate the blackness caused by tannins and restore the pH level of the water.

3. Apply Seachem Purigen

Purigen is another product helpful in eliminating the discoloration effect of tannins in aquarium water.

Purigen works by targeting all the impurities, soluble or insoluble, to get rid of them and leave water that is crystal clear. 

Below are the steps for using Purigen.

  1. Place the Purigen in the right bag.
  2. Put the bag containing the Purigen in the filter. You can put the product in the Canister, Media Chamber, or Box filters.
  3. Observe the flow. To achieve excellent results, place the Purigen bag against the highest flow of water in the filter.

Besides that, you can use an aquarium water conditioner to get the right conditions for your fish or aquatic life in general.


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