Tannins & Humic Substances

You add a beautiful piece of driftwood, and the next morning your crystal-clear water looks like a cup of tea. Welcome to the world of tannins.

What Are Tannins?

Tannins are a class of polyphenolic compounds found in bark, leaves, seed pods, and wood. When organic material is submerged, these compounds leach into the water, staining it amber to dark brown.

Common Sources in Aquariums:

• Driftwood: Mopani, Malaysian, Spider Wood — all leach tannins.
• Indian Almond Leaves (Catappa): Deliberately added for their tannin-rich properties.
• Alder Cones: Small seed cones that release tannins slowly.
• Peat Moss: Used in filters to soften water and release humic acids.

The Chemistry

Tannins are part of a broader group called Humic Substances, which include:

1. Humic Acid: Large molecules that lower pH and bind heavy metals.
2. Fulvic Acid: Smaller molecules that act as natural chelators, making minerals bioavailable.
3. Tannins: The visible staining compounds.

Effects on Water Parameters

• pH: Tannins are mildly acidic. They will lower pH over time, especially in soft water (low KH).
• KH: They consume KH buffer slowly.
• Color: Amber to dark brown. Harmless but cosmetic.
• Antibacterial: Tannins have mild antifungal and antibacterial properties. Many breeders use them to protect eggs.

Blackwater Biotopes

In the Amazon, rivers like the Rio Negro are naturally stained dark by decomposing leaf litter. Fish from these habitats (Discus, Cardinal Tetras, Chocolate Gouramis) have evolved to thrive in tannin-rich, acidic, soft water.

Recreating this in an aquarium is called a Blackwater Biotope:

• pH: 4.5 – 6.0
• GH: 0 – 3 dGH
• Color: Deep amber/brown
• Lighting: Dim, dappled

Removing Tannins

If you don't want the tea-stained look:

1. Pre-soak Driftwood: Boil or soak wood in a bucket for days/weeks before adding it.
2. Activated Carbon: Place in your filter — it absorbs tannins and clears water within days.
3. Purigen: A synthetic resin (by Seachem) that strips organics and polishes water to crystal clarity.
4. Water Changes: Dilution works, but the wood will keep leaching.

When Tannins Are Beneficial

• Stress Reduction: The darker water makes shy fish feel secure (mimics natural habitat).
• Breeding: Tannins protect eggs from fungal infection.
• Heavy Metal Binding: Humic acids chelate copper, zinc, and lead, reducing their toxicity.