Brown algae in fish tank: what it is, why it happens and how to get rid of it

Brown algae in a fish tank is almost always diatoms: tiny single-celled algae that coat the glass, sand and leaves with a brown, dusty film. It's harmless to fish, very common in new tanks, and usually fades on its own within a few weeks as the tank settles.

You can wipe it off at any time, and snails and small algae-eating fish will help. If it keeps coming back in an older tank, look at your light, your water source and how much waste is building up.

How to identify it

Brown or golden-brown film on glass, sand, rocks, decor and plant leaves.
Soft and dusty or slightly slimy. It wipes off with a finger or cloth in one pass and leaves clean glass.
Often patchy, heaviest in dim corners and on the sand.
Appears in the first weeks after a tank is set up, or after a big change.

What diatoms are: each diatom builds a tiny shell out of silica, which is the same material as glass and sand. They take dissolved silica (silicate) from the water to do it.

Not diatoms: dark bristly tufts that won't wipe off are black beard algae. Dark green or blue-green slime that peels off in sheets and smells is cyanobacteria. Hard green spots on glass are green spot algae. Reddish-brown water (not film) is usually tannins from driftwood, which is harmless.

When it's a problem

Normal: a brown film in a tank less than about two months old. Nearly every new tank goes through this.
Early problem: it comes back within a few days of wiping in an older tank, or it covers plant leaves.
Serious: it's still heavy after three months, or it's smothering small plants.
Emergency: never, for diatoms alone. But a brand-new tank also has a risk of ammonia and nitrite spikes, so test the water if fish look unwell.

What to do right now

Relax. Diatoms don't harm fish.
Test ammonia and nitrite, especially in a tank under eight weeks old. Diatoms are a sign the tank is still young. See cycling.
Wipe the glass with an algae pad or magnetic cleaner. Use a soft, clean sponge on acrylic.
Siphon the film off the sand during a water change. Stir the top layer gently with the siphon.
Gently rub leaves between your fingers to clear them.
Do a water change of 20 to 30% with dechlorinated water at the same temperature.

What to do next

Weeks 1 to 6: let the tank mature. In most new tanks, diatoms fade as the filter matures, plants grow and other algae and bacteria take over the surfaces. Keep doing weekly water changes and wiping the glass when it bothers you. Avoid scrubbing hard every day; it just stirs it around.

Add a clean-up crew, once the tank has cycled.

Nerite snails: excellent at cleaning glass, rocks and wood. They can't breed in freshwater, so they won't overrun the tank (though they may leave small white eggs on hard surfaces).
Otocinclus: tiny, peaceful catfish that graze diatoms off leaves and glass. Keep them in groups of at least four to six, and only add them to a fully cycled tank, as they're sensitive to ammonia. Once diatoms are gone, feed them algae wafers or blanched vegetables, or they can starve.
Amano shrimp: clean diatoms off plants and fine leaves.

If it keeps coming back after two to three months:

Check your light. Diatoms do well in low light. Many planted tanks see them drop once light and plant growth improve. In unplanted tanks, keep lights to 6 to 8 hours a day.
Check your water. Some tap water has a lot of silicate. Your water supplier's report may list it, or you can test for it. Using some reverse osmosis (RO) water, or a silicate-removing filter media, can help.
Check your substrate and decor. Some sands, gravels and rocks keep releasing silicate for a while. Rinse new substrate well before using it.
Reduce waste. Vacuum the gravel, don't overfeed, and keep the filter clean. See how to clean a fish tank.

Don't use algae-killing chemicals for diatoms. They aren't needed, and they can harm plants, shrimp and snails. If you ever do medicate a tank for another reason, remove activated carbon first and take extra care with shrimp, snails, scaleless fish, catfish and loaches.

Products that help

Magnetic glass cleaner or algae pad: wipes diatoms off glass in seconds. Choose an acrylic-safe one for acrylic tanks.
Gravel vacuum: lifts the brown film off sand and removes waste at the same time.
Silicate-removing filter media: only worth it if your tap water is high in silicate and diatoms won't go.
RO water (shop-bought or from a home unit): low in silicate. Mix with tap water and add minerals back as needed.
Light timer: keeps the light period steady.
Algae wafers: food for otocinclus and snails once the diatoms are gone.
Keep on hand: a test kit for ammonia, nitrite and nitrate, and dechlorinator.

How to prevent it

Expect it in new tanks, and don't add lots of fish while the tank is still young.
Rinse new sand and gravel thoroughly.
Keep regular water changes and gentle gravel cleaning going.
Keep a few nerite snails or otocinclus in established tanks.
Grow healthy plants that compete for nutrients.

Why it happened

New tanks have spare silicate (from tap water, new sand and decor), some ammonia, and few competitors, which is perfect for diatoms. As the filter matures and plants and other algae get established, diatoms usually lose out. In older tanks, they tend to return with low light, high silicate in the water, or a build-up of waste.

When to get help

Ask for help if fish are gasping or acting unwell (that points to water quality, not algae), or if a brown film persists for months despite good care. A local fish store can test your water, and your water supplier can tell you about silicate.

In the game

Multiple Tank Syndrome doesn't have a brown algae card yet. The related card is "Algae takeover", which only an algae eater can clear.

Sources

University of California Museum of Paleontology. Diatoms: more on morphology (silica shells).
Green Aqua. Silicate in the water: brown algae (diatoms) in the aquarium.
Merck Veterinary Manual. Management of aquarium fish (a new tank's biofilter can take up to 8 weeks to establish).