Tannins in aquarium water: why your fish tank water is yellow or brown, and what to do

If your fish tank water has turned yellow or tea-brown but stays clear, the cause is almost always tannins leaching from driftwood, leaves or peat. Tannins are harmless to fish at aquarium levels, and many fish from blackwater rivers actually do better in stained water.

You don't have to do anything. If you don't like the colour, activated carbon and water changes clear it within days, and soaking new wood first stops most of it. The one thing to rule out is brown water that is also cloudy or smelly, because that points to rotting food or a dead fish, not tannins.

How to identify it

The colour: pale yellow, amber or strong tea, but the water is clear. You can see through it like a glass of tea.
The timing: it appears within days of adding new driftwood (especially mopani or Malaysian wood), Indian almond (catappa) leaves, oak or beech leaves, alder cones, or peat. It comes back after water changes.
The smell: clean and earthy, or no smell at all.
The test: fill a white cup or bucket with tank water. Tannins look like weak tea. Clean water looks colourless.

Some cheap flake foods with heavy food colouring can also tint water yellow. If there is no wood or leaves in the tank, check your food.

When it's a problem

Normally it isn't. Tannic acid is not a poison at the levels wood and leaves release. Wild blackwater rivers like the Rio Negro are naturally stained dark by tannins and other plant compounds, and fish such as cardinal tetras, many Apistogramma, wild bettas, discus and angelfish live in that water. Many keepers add leaves on purpose because the fish look calmer and colour up under stained, dim water.

Tannins do lower pH slightly. In soft water with little buffering (low KH, see pH) the drop can be noticeable. In normal or hard tap water it is usually tiny, often a tenth of a pH unit or so. That only matters if you keep hard-water fish such as African rift-lake cichlids or livebearers in very soft water.

When brown water is NOT tannins

Brown and cloudy: suspended waste or a bacterial bloom, often from overfeeding. See cloudy water.
Brown and smelly (rotten, sour or sewage smell): rotting food, a decaying plant, or a dead fish hidden behind rocks or in the filter. See smells and dead fish.
Brown with fish gasping: test for ammonia right away. See ammonia and gasping.
Brown dust on glass and decor, not in the water: that's diatoms. See brown algae.

What to do right now

Check that the water is clear and smells clean. If so, relax: it's tannins.
Test pH and compare it with your tap water. A small drop is normal.
If the water is cloudy, smelly or the fish look unwell, test ammonia and nitrite, search the tank for anything dead, and do a 30–50% water change.
Decide whether you want to keep the colour. If you keep blackwater fish, you may want to leave it.

What to do next

To clear the colour

Activated carbon: put a fresh bag in the filter, after the sponge. It pulls tannins out quickly, often in a day or two. Carbon fills up fast with organics, so replace it when the colour starts creeping back, which may be days or weeks.
Synthetic adsorbent resin: a reusable pouch that removes the brown colour and can be recharged. It is gentler on liquid plant fertiliser than carbon.
Water changes: weekly 25–50% changes dilute the colour. Alone they are slow, because the wood keeps leaching.
Take the source out and soak it: if one piece of wood is the culprit, remove it and soak it in a bucket, changing the water every day or two, until the bucket water stays nearly clear.

Cautions

Remove carbon when you medicate, because it can take some medicines out of the water.
Shrimp and snails are fine with tannins, but watch pH and hardness in very soft water. Snails need some hardness for their shells.
Plants: dark tea-coloured water cuts light. Light-hungry plants may slow down. Low-light plants such as Java fern, Anubias and Cryptocoryne don't mind.
Hard-water fish: if you keep mbuna or livebearers, avoid heavy wood and leaves, or keep carbon running. See adjusting pH.

To keep tannins on purpose

Add a few dried, naturally fallen leaves (Indian almond, oak or beech) and judge by eye. Aim for a light tea tint. If it gets darker than you like, change some water. Collect leaves away from roads and sprayed areas, and never boil them.

Products that help

Activated carbon: removes tannins and colour quickly; replace it regularly.
Synthetic adsorbent resin: removes colour and organics, rechargeable, kinder to fertilisers.
Liquid pH and KH test kit: shows whether the tannins are moving your pH.
A spare bucket or tub: for soaking new wood before it goes in.
Dried botanical leaves: if you want a blackwater look on purpose.

How to prevent it

Soak new wood for 1–3 weeks in a tub, changing the water every day or two until it barely stains. This also helps the wood sink.
Peel off loose bark, which holds a lot of tannin.
Boiling is optional. Many keepers boil small pieces to speed things up. Others point out that boiling only clears the outer layer and softens the wood, so a long soak does the same job. Either way, dense dark woods like mopani can stain for months.
Choose low-tannin wood (spider wood, manzanita, or well-aged pieces) if you want clear water.
Keep a bag of carbon running for the first month after adding new wood.

Why it happened

Wood and dead leaves contain tannins and related plant compounds that dissolve slowly in water. New wood releases the most, and the amount drops over weeks to months. The colour is the same thing that makes tea brown.

When to get help

Ask an experienced keeper or your local fish store if the water is brown and cloudy or smelly, if fish are gasping or dying, or if pH has crashed below what your fish can handle. Those are signs of something other than tannins.

In the game

Multiple Tank Syndrome doesn't have a tannins card yet, but the Indian almond leaves plant card and the Blackwater 40 tank capture the same idea: stained, soft, slightly acidic water that blackwater fish love.