Tannins and tanks: what does it actually mean?

What are tannins?

Tannins (tannic or humic acids) are natural compounds in wood, bark, leaves, and peat. They leach into water and tint it yellow, amber, or brown — like weak tea. In nature, forest streams and flooded areas get this colour from decaying plant matter. In aquariums, driftwood, Indian almond leaves, peat, and some substrates release tannins into the water.

Where do tannins come from in a tank?

• Driftwood and bogwood: The most common source. New wood leaches heavily; it slows over weeks or months.
• Indian almond leaves (catappa): Often used in betta and blackwater tanks. Release tannins as they break down.
• Peat: In filter media or substrate, peat softens water and adds tannins.
• Oak leaves, alder cones, botanicals: Hobbyists add these deliberately for a natural look and mild antibacterial effect.

Are tannins harmful?

No. Tannins are not toxic to fish, plants, or invertebrates. Many species — tetras, rasboras, discus, bettas, some cichlids — come from tannin-stained habitats. Slightly tinted water can reduce stress and mimic their natural environment. Tannins may also slightly lower pH and soften water, which some fish prefer.

Blackwater tanks

A blackwater tank deliberately uses wood, leaves, and botanicals to create dark, tea-coloured water. The goal is to replicate Amazon, Southeast Asian, or African blackwater habitats. Fish from those regions often thrive in such setups. It’s an aesthetic and husbandry choice, not a problem.

If you want clearer water

Soak or boil wood before adding. Use water changes and activated carbon in the filter to remove or dilute tannins. Over time, most wood stops leaching as much. See adding wood: water changing colour for detailed steps.

Quick takeaways

• Tannins tint water yellow/brown. They come from wood, leaves, peat, botanicals.
• They are harmless; many fish come from tannin-rich habitats.
• Blackwater tanks use tannins on purpose. To reduce: soak wood, use carbon, do water changes.