A few weeks after setup, a brown film starts appearing on the glass, the sand, decorations and even plant leaves. It wipes away with a finger and comes back within days. Almost every new tank goes through this, and it is usually not a sign that anything is wrong.
That film is most often diatoms, a group of single-celled algae with silica shells. Here is how to identify it, what feeds it, and how to manage it without overreacting.
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How to identify diatoms
Diatoms look like a thin coating of brown or golden-brown dust. Key signs:
- Easy to wipe off glass with a finger or soft scraper
- Coats flat surfaces: sand, gravel tops, rocks, slow-growing leaves
- Appears in tanks a few weeks to a few months old
- Comes back quickly after cleaning during the early phase
| Brown growth | Texture | Typical setting | Likely ID |
|---|---|---|---|
| Dusty brown film, wipes off easily | Powdery, flat | New tank | Diatoms |
| Brown slime sheets, often with bubbles | Slimy, peels as a sheet | Low-flow areas; any tank age | Possibly cyanobacteria (often more red or blue-green, but can look brown) |
| Short dark tufts on leaf edges or wood | Firmly attached, bristly | Established planted tanks | Black beard algae (dark gray to black) |
| Brown on driftwood, fuzzy white or translucent | Soft, cottony | New driftwood | Biofilm/fungal growth on wood, usually harmless |
Cyanobacteria is the main lookalike to rule out. It tends to form a slimy mat that peels off in sheets and can have a distinct musty smell. Diatoms are drier and dustier.
Why diatoms show up in new tanks
The commonly cited explanation is a combination of factors that are typical of young aquariums:
- Silicates from tap water, some sands, and new substrates, which diatoms use to build their shells.
- Spare nutrients while the biological filter and plants are not yet established.
- Few competitors. In a mature tank, plants and other algae compete for the same resources.
Low light is sometimes blamed, but diatoms also appear under bright light. The age of the tank is the most consistent common factor.
How to get rid of diatoms
- Wipe and remove. Use a soft algae pad or magnet cleaner on glass, and siphon the top layer of sand during water changes. Removing the film physically takes the nutrients out with it.
- Keep up partial water changes. Regular changes reduce dissolved nutrients and help the tank stabilize. Our guide on water change frequency has schedules.
- Feed modestly. Extra food breaks down into extra nutrients.
- Keep a consistent light schedule on a timer rather than leaving lights on for long, irregular periods.
- Add a cleanup crew once the tank is cycled. Many snails and some fish graze diatoms readily (see below).
- Be patient. In most tanks it fades within a few weeks as the system matures.
Cleanup crew options for diatoms
Only add animals after the tank has finished cycling. Grazers need stable water like any other livestock, and some need an established tank with natural biofilm to feed on.
Nerite snails
Widely recommended diatom grazers. They do not reproduce in freshwater, though they may leave small hard eggs on surfaces.
Otocinclus
Small grazing catfish often recommended for diatoms. Commonly advised for established tanks and kept in groups. Check a care sheet first.
Amano and cherry shrimp
Graze biofilm and soft algae. Best in tanks without fish large enough to eat them.
Ramshorn snails
Eat soft films and leftover food. They reproduce readily, which some keepers enjoy and others do not.
For more choices, see our freshwater cleanup crew guide. A grazer is a helper, not a replacement for addressing nutrients.
Once diatoms fade, grazers can run out of food. Plan to supplement with algae wafers or blanched vegetables as recommended for the species.
If diatoms will not go away
A tank older than several months that still grows heavy diatoms deserves a closer look.
- Test your source water for silicate. Some regions have high silicate tap water. Silicate test kits exist, and some water utilities publish it in their reports.
- Consider RO or a silicate-removing resin if silicate is high and the problem is ongoing. See tap vs RO water.
- Check substrate. Some sands may contribute silicates; this usually diminishes over time.
- Review lighting. Very long photoperiods or old bulbs that have shifted spectrum can favor algae in general.
- Look for overfeeding and high nitrate or phosphate readings.
Cleaning diatoms without damaging the tank
Diatoms are easy to remove, but the tools you use matter, especially on acrylic tanks and delicate plants.
| Surface | Best tool | Watch out for |
|---|---|---|
| Glass walls | Magnet cleaner, soft pad, or a blade scraper | Sand caught under a magnet pad can scratch; rinse pads often |
| Acrylic walls | Acrylic-safe pad or plastic card | Metal blades and many glass pads scratch acrylic |
| Sand surface | Gravel vacuum hovered just above the sand | Pushing deep into fine sand sucks it up the tube |
| Plant leaves | Gentle rub between fingers, or let grazers handle it | Tearing soft leaves; aim to remove only the film |
| Rocks and decor | Soft toothbrush reserved for the tank | Never use household cleaners or soap |
Do your wiping just before a water change, so the dislodged film gets siphoned out instead of resettling. If you use a filter with a fine mechanical stage, it will catch some of what floats free; rinse that stage in removed tank water, not tap water, so you do not harm the beneficial bacteria. Our guide to algae scrapers and magnet cleaners compares the tool types.
A simple weekly routine during the diatom phase
- Test ammonia, nitrite and nitrate, since the tank is still young
- Wipe the front glass and any heavily coated leaves
- Hover-siphon the sand surface during a partial water change
- Check that the light timer is still set to your chosen schedule
- Note whether coverage is shrinking week to week; it usually does
Tracking it in a notes app or a simple log helps you see progress that is hard to notice day to day, and it gives you real data if you later need to troubleshoot.
Is diatom algae harmful?
Before answering, it helps to be clear about what diatoms are not. They are not a disease, they are not a sign of poor filtration in a young tank, and they are not something you need chemical algaecides for. Algaecides can harm invertebrates and plants, and they do nothing about the conditions that let algae grow. Physical removal and patience are the standard advice for good reason.
Diatoms are generally considered harmless to fish. They are an eyesore and can coat slow-growing plant leaves enough to shade them, which is why wiping or grazing helps in planted tanks. The bigger concern is what they signal: a young tank still settling in. That is a good reason to stock slowly and keep testing ammonia and nitrite while the tank matures.
FAQ
How long do diatoms last in a new tank?
It varies, but many tanks see diatoms fade within a few weeks to a couple of months as the tank matures, plants grow in, and competitors establish.
Are diatoms bad for fish?
They are generally considered harmless to fish. Heavy coverage can shade plant leaves, so wipe or siphon it away and keep up water changes.
What eats brown diatom algae?
Nerite snails, otocinclus, and many shrimp are commonly recommended. Add them only after the tank is cycled, and check each species' care requirements first.
Should I turn the lights off to kill diatoms?
A long blackout is not usually necessary. A consistent, moderate photoperiod on a timer plus physical removal and water changes is the more reliable approach.
The bottom line
Brown diatom algae is a normal stage in most new aquariums. Wipe it, siphon it, feed lightly, keep the light schedule consistent, and add grazers once the tank is cycled. If it outlasts the first few months, investigate silicate and nutrients. Browse more troubleshooting help on the AquariumSetup blog.