Mold on driftwood: the white fuzz on new aquarium wood, and when to worry

The white, cottony fuzz or clear slime that grows on new driftwood is almost always harmless. It is a biofilm of bacteria and water moulds feeding on sugars and other compounds leaching out of the wood, and it usually fades on its own within one to three weeks, though on dense wood it can take five or six.

Your fish won't be hurt by it, and shrimp, snails and many plecos will happily eat it. You only need to worry if the wood is soft and crumbling, the smell keeps getting worse, or the fuzz is still coming back after a couple of months.

How to identify it

White or grey fuzz that looks like cotton wool or cobweb, on the surface of new wood.
Clear or white slime that wobbles in the current and comes away in strings.
Timing: it shows up within days to a couple of weeks of adding new wood, or wood that has been dried out and re-soaked.
Smell: none, or a mild earthy or mushroomy smell close up.

It's different from fungus on fish (cotton-like tufts on a fish's body), which is a fish health problem, and from brown algae, which is a brown dust that wipes off.

When it's a problem

Normal: fuzz or slime on wood that has been in the tank less than about six weeks, with clear water and normal fish.
Keep an eye on it: heavy growth that clogs the filter intake, smothers moss or plants tied to the wood, or makes the water slightly cloudy.
A problem: wood that is soft, spongy or crumbling; a strong rotten smell that gets worse over time; growth that keeps returning after two or three months; or cloudy, smelly water with rising ammonia. That points to rot, and the wood should come out.

What to do right now

Check your fish. If they are eating and behaving normally, there's no rush.
Test ammonia and nitrite if the water is cloudy or smells, just to be sure (see ammonia).
Press the wood with a fingernail. Hard wood is fine. Soft, crumbling wood may be rotting.
Leave it, or siphon it. You can suck the fuzz off with your gravel vacuum during a water change, or wipe it with a clean, soap-free toothbrush.

What to do next

Let nature clean it

The growth feeds on what the wood releases, so it dies back as the wood "settles in." Most keepers find it gone within a few weeks. Expect it to grow back once or twice if you wipe it off; that's normal and doesn't mean anything is wrong.

Clean-up crew

Shrimp, especially Amano and cherry shrimp, graze it eagerly. Many shrimp keepers value new wood as extra food for baby shrimp.
Snails such as nerites and Malaysian trumpet snails eat it.
Plecos and otocinclus graze biofilm on wood. Many plecos rasp at wood as part of their diet (see feeding plecos).

Removing it faster

Take the wood out and scrub it under running tap water with a stiff brush, then put it back.
For small pieces, pour boiling water over them, or boil them for 15–20 minutes, then let them cool.
Don't use bleach or soap on wood. Wood soaks up chemicals and can release them slowly into your tank.

Cautions

Moss and plants attached to the wood can be smothered by heavy growth; brush it gently away from them.
Filter intakes: clean the intake sponge if strands of slime collect there.
Shrimp tanks: no treatment needed; the shrimp will do the job.

When the wood is rotting

Remove wood that is soft and crumbling or that keeps smelling worse. Wood collected from land is more likely to rot than wood that has already spent years in water. Softwoods with fresh sap (pine, cedar) and wood with an oily or chemical smell should never go in a tank.

Products that help

Stiff, soap-free brush: for scrubbing new wood.
Large tub or bucket: for pre-soaking new wood before it goes in.
Gravel vacuum: lifts fuzz out during water changes.
Liquid test kit: confirms water is fine if you're unsure.
Pre-filter sponge on the filter intake: stops slime strands clogging the filter.

How to prevent it

Soak new wood for one to two weeks in a tub of water, changing it every few days. This washes out much of what the biofilm feeds on and reduces tannins.
Boil small pieces if you like. Experts disagree on how much it helps: some say boiling speeds things up and kills hitchhikers, others say it only treats the surface and softens the wood. A long soak works either way.
Scrub off loose bark and soft patches before soaking.
Buy aquarium wood from a store, or collect only dead, long-weathered hardwood with no sap smell from clean water or dry ground away from roads and spraying.
Add new wood before the fish in a new setup, so the bloom passes before they arrive.

Why it happened

New wood releases sugars, starches and other food for microbes. Bacteria and water moulds in every aquarium find that food and bloom on the surface. As the easily dissolved material runs out, the bloom fades.

When to get help

Ask an experienced keeper or your local fish store if the growth is still strong after two months, if the wood smells foul, or if fish start gasping or getting sick at the same time. Those signs point to rot or a water-quality problem rather than harmless biofilm.

In the game

Multiple Tank Syndrome doesn't have a driftwood card yet. The Clean-up crew goal (five kinds of bottom-dweller such as shrimp, snails and plecos) is the closest: those are exactly the animals that eat driftwood fuzz.