01
What is brown algae in a fish tank?
What most fishkeepers call brown algae is a film of diatoms, microscopic algae with cell walls made of silica. They form a soft brown or golden-brown dust on glass, sand, rocks, decor and slow-growing leaves. Run a finger or a sponge over it and it comes off in a smear, which is the easiest way to tell diatoms from tougher algae.
Diatoms are a normal part of a young aquarium. In the first weeks after setup, while the nitrogen cycle is still settling, there is little competition from plants or other algae, and the water and new substrate often contain silicates. Diatoms thrive in those conditions, then usually retreat as the tank matures.
They are not harmful to fish, shrimp or snails. The main issues are looks and, on plants, a film that blocks some light. If you find brown algae in a new tank, it is typically a sign that the tank is going through a normal stage, not that something is wrong.
02
Is it diatoms or something else?
Before you change anything, make sure you are dealing with diatoms. A few other growths can look brown or dark.
| Growth | Appearance | Texture and smell | Usual cause |
|---|---|---|---|
| Diatoms (brown algae) | Brown to golden dusty film on glass, sand, leaves | Soft, wipes off easily, no smell | New tank, silicates, spare nutrients |
| Cyanobacteria | Dark green, blue-green or reddish slimy sheets | Slimy, peels off in sheets, musty smell | Low flow, high organics |
| Black beard algae | Dark gray to black tufts | Firm, will not wipe off | Unstable CO2, organics |
| Green spot algae | Hard green dots on glass and slow leaves | Hard, needs a scraper | Strong light, low phosphate |
| Biofilm | Clear to whitish film or jelly on new wood | Soft, harmless | New driftwood, organics |
If the "brown algae" is slimy, smells musty and comes off in sheets, it is more likely cyanobacteria. If it forms firm black tufts, see our guide to black beard algae.
03
What causes brown algae (diatoms)?
Diatoms are opportunists. Several factors usually combine:
- A new tank. Diatoms are classic in the first 2–8 weeks, during and just after cycling. See our page on new tank syndrome for the other changes happening at that stage.
- Silicates. Some tap water is rich in silica, and new sand, glass beads and some substrates release it too. Diatoms use it to build their cell walls.
- Spare nutrients. Overfeeding, a high fish load and infrequent water changes leave nitrate and phosphate for algae to use.
- Low or poor light. Diatoms cope with dim light better than most plants, so they often appear in tanks with weak lighting or low plant mass.
- Low plant growth. Few plants, or plants still settling in, means little competition for nutrients.
04
How to get rid of brown algae step by step
In most tanks, a short cleaning routine plus patience is all it takes. Follow these steps:
- Wipe the glass with an algae magnet, a soft pad or a scraper. Diatoms come off easily, so there is no need for force.
- Gently brush leaves with a soft toothbrush or rub them between your fingers. Rinse decor and rocks in a bucket of tank water.
- Vacuum the brown layer off the sand during your next water change with a good gravel vacuum. Stir the surface lightly as you go.
- Change 25–30% of the water weekly, using a dechlorinator, and do not overfeed.
- Keep the photoperiod at 6–8 hours a day, on a timer for consistency.
- Once the tank is cycled, add a cleanup crew suited to its size (see below).
- Be patient: in a new tank, diatoms usually fade within a few weeks once plants grow and the biofilter matures.
Do not reach for algaecides for diatoms. They are rarely needed, can harm plants and invertebrates, and dead algae add more waste to a tank that is still settling. Our tank cleaning guide covers safe routine cleaning.
05
Cleanup crew that eats diatoms
Several popular algae eaters love diatoms. Add them only to a cycled tank, and only if there is enough algae or supplementary food, since a spotless tank can leave them hungry.
- Otocinclus: small, peaceful catfish that graze diatoms from glass and leaves. Keep them in groups of 5 or more in a mature tank of at least 10–20 gallons (38–75 L), and supplement with blanched vegetables or algae wafers.
- Nerite snails: excellent diatom grazers that will not breed in freshwater. Their eggs can leave small white dots on hardscape.
- Amano shrimp and cherry shrimp: pick diatoms off plants and decor.
- Bristlenose pleco: a good option for larger tanks of 25–30 gallons (95–115 L) and up.
- For more choices, see the best snails for algae.
06
Brown algae in an established tank
If brown algae keeps returning in a tank that is several months old, look for a lasting cause rather than a phase. Test your tap water and tank with a reliable aquarium test kit for nitrate and phosphate, and, if you suspect silica, a separate silicate test.
High nitrate points to overfeeding, overstocking or too few water changes: our guide on how to lower nitrates walks you through fixes. If your tap water is high in silica, a silicate-absorbing filter medium such as Seachem PhosGuard (which binds phosphate and silicate) or mixing in reverse osmosis water can help. Follow the manufacturer’s directions for the amount.
Weak lighting can also favor diatoms over plants. An upgrade from our aquarium light reviews gives plants the edge, as long as you keep the photoperiod reasonable. Too much light simply swaps diatoms for green algae, as in green water.
07
When to worry
Diatoms alone are not a reason to worry. Pay attention if brown growth comes with other signs: fish gasping or hiding, a sudden spike in ammonia or nitrite, a bad smell, or slimy sheets spreading fast. Those point to a water-quality or cyanobacteria problem rather than diatoms. Test the water, do a water change, and check our aquarium troubleshooting hub for the next step.
Frequently asked questions
Frequently asked questions
Will brown algae go away on its own?
In a new tank, usually yes. Diatoms are most common in the first 2–8 weeks and fade as the biofilter matures, plants grow and silicates are used up. Keep wiping the glass, vacuuming the sand and changing water weekly, and avoid overfeeding. If brown algae persists after three months, test nitrate, phosphate and silicate to find a lasting cause.
Is brown algae bad for fish?
No. Diatoms are harmless to fish, shrimp and snails, and many algae eaters actively feed on them. They are mostly a cosmetic issue, though a thick film on leaves can reduce the light plants get. Just make sure what you see is really diatoms and not slimy cyanobacteria, which signals a different problem.
Why does brown algae appear in a new tank?
A new tank offers diatoms ideal conditions: silicates from tap water and fresh substrate, spare nutrients while the nitrogen cycle settles, and little competition from plants. Brown algae in a new tank is a normal stage that most aquariums go through during the first weeks.
What eats brown algae in a fish tank?
Otocinclus catfish, nerite snails, Amano and cherry shrimp, and bristlenose plecos all graze diatoms. Add them only once the tank is cycled, match them to your tank size and offer supplementary food, because they can go hungry once the diatoms are gone.
Should I turn off the lights to kill diatoms?
A blackout is not necessary for diatoms, and they tolerate low light well. A steady 6–8 hour photoperiod on a timer, regular cleaning and good plant growth work better. Blackouts are sometimes used against green water or cyanobacteria, not routine diatom films.
Sources and references
Written and reviewed by the handsoffmyhobby editorial team according to our editorial policy.
