Your new tank is three weeks old, everything seemed fine, and now every surface is coated in a rusty-brown dust that smears when you touch it. Congratulations — you have diatoms, the most normal "problem" in the hobby. Brown diatom algae in new tanks is so predictable it might as well be a milestone: it shows up, it looks awful, and then — if you leave it alone — it leaves on its own. Here's why it happens and the few things worth doing while you wait.
What Diatoms Are (They're Not Really Algae)
Diatoms are single-celled organisms with glass-like silica shells, technically distinct from true algae. In the aquarium they form a soft, dusty brown coating on glass, substrate, hardscape, and plant leaves. The coating wipes off effortlessly — a finger swipe leaves a clean streak — and it has no slimy or gritty texture.
Key identification points:
- Color: golden-brown to rusty brown, never green or black.
- Texture: powdery dust, not spots, threads, or slime.
- Timing: overwhelmingly a new-tank phenomenon, typically weeks 2–8.
- Smell test: none — unlike cyanobacteria, which smells swampy (see our cyanobacteria treatment guide if there's an odor).
Why New Tanks Get Diatoms
Silicates: the fuel
Diatoms build their shells from dissolved silicates, and new tanks are full of them. Fresh tap water contains silicates; new sand and gravel leach them; fresh glass is literally silica. Until the tank's biology and your water changes draw silicate levels down, diatoms have an all-you-can-eat buffet.
Instability: the opportunity
New tanks have immature bacterial colonies, fluctuating ammonia, unsettled plant growth, and inconsistent CO2 — a buffet of instability that opportunistic organisms exploit. Diatoms are pioneer species: they colonize disturbed environments fast, before the more complex ecosystem (plants, biofilm grazers, competing algae) establishes and outcompetes them.
Low light tolerance
Unlike most algae, diatoms thrive in low light, so dim new-tank lighting (often run cautiously during cycling) doesn't slow them at all. This is why they can blanket a tank that gets barely any light — they're not playing by the usual rules.
The Timeline: What to Expect
- Weeks 2–4: First dusting appears, usually on the glass nearest the light and on the substrate. Spreads steadily.
- Weeks 4–8: Peak ugliness. Everything brown. This is when most beginners panic and reach for chemicals.
- Weeks 8–12: Fades as silicates deplete, plants establish, and grazers multiply. Often disappears quite suddenly — clear one morning after weeks of brown.
This timeline assumes a normally progressing tank. Tanks that are perpetually "new" — constant rescapes, filter sterilization, chronic instability — can host diatoms indefinitely, because they never leave the pioneer stage.
What to Do (and What Not to Do)
Do: wipe the glass, gently
Unlike green dust algae, diatoms can be wiped freely — they don't have the same spore-scattering lifecycle trick. Use a soft algae pad on the front glass whenever it bothers you. Siphon the dislodged dust during water changes.
Do: keep up water changes
Weekly 30–50% water changes steadily dilute silicates — this is the single most effective action. Each change physically removes both diatoms and their fuel. This matters more than any product.
Do: add grazers once the tank is cycled
Otocinclus catfish and nerite snails devour diatoms enthusiastically — a small crew can keep glass and leaves visibly cleaner during the peak weeks. Only add them once ammonia and nitrite read zero; they're sensitive to cycling water. Shrimp graze diatoms too, though less aggressively.
Do: keep plants growing
Healthy, growing plants compete with diatoms for nutrients and eventually shade them out. Don't stall the tank waiting for the diatoms to pass — plant heavily from the start, dose fertilizer normally, and keep CO2 stable. Stalled plants prolong the diatom phase; see why plants stop growing if yours have flatlined.
Don't: dose silicate removers routinely
Phosphate/silicate-absorbing resins work, but they're a crutch — the tank depletes silicates naturally within weeks. Save your money unless your tap water is extraordinarily high in silicates (test if diatoms persist past month four).
Don't: use algaecides
Diatoms in a cycling tank plus algaecide is a recipe for crashing the bacterial colonies you're trying to grow. The "cure" is worse than the brown dust, which was leaving on its own.
Don't: black out or tear down
Diatoms tolerate low light, so blackouts barely dent them. And tearing down a new tank over its most normal phase just restarts the clock — the next setup will get diatoms too.
Binds silicates during water changes while the tank matures past diatoms: Seachem Prime.
Diatoms on Plant Leaves
A brown dusting on leaves looks alarming but is rarely harmful short-term. Gently rub leaves between your fingers during water changes, or let otos and shrimp do it. Trim only leaves that are fully smothered and clearly dying underneath — most leaves shed the dust once grazers and water changes catch up.
One exception: delicate carpeting plants can be smothered if the diatom layer gets thick enough to block light for weeks. If your carpet is disappearing under brown dust, siphon gently over it with a turkey baster during water changes rather than scrubbing — carpets tear easily. Our carpet plants not spreading guide covers related establishment problems.
Wipes diatom dust off glass and hardscape without disturbing new plants: FL!PPER magnetic cleaner.
When Diatoms Aren't Normal
Diatoms past the new-tank window deserve investigation:
- Persisting beyond 3–4 months — test your tap water for silicates. Some municipal supplies are very high; a reverse-osmosis unit or silicate-absorbing media may be warranted.
- Sudden diatoms in an established tank — something destabilized the system: a major rescape, new silica sand added, filter media replaced all at once, or a long-neglected tank suddenly cleaned aggressively. Treat it as a mini-cycle and stabilize.
- Diatoms plus plant melting — the diatoms may be secondary to a bigger instability (ammonia event, CO2 failure). Check our melting diagnosis guide if plants are dissolving alongside the brown dust.
Diatoms and Your Substrate Choice
Not all substrates feed diatoms equally. Silica-based sands (play sand, pool filter sand, blasting sand) leach the most silicates early on — they're the classic diatom fuel, though they still settle down within weeks. Commercial aquarium soils and baked-clay substrates leach far less. This doesn't mean you should avoid sand — it's cheap, natural-looking, and perfectly fine long-term — just expect a slightly heavier diatom phase in the first month and compensate with diligent water changes.
One practical trick: rinse new sand thoroughly before adding it. Rinsing won't remove all silicates, but it washes away the fine silica dust that dissolves fastest, noticeably softening the initial diatom bloom. For aquasoil users, the bigger early-tank concern is usually ammonia leaching rather than silicates — if your plants are melting in fresh aquasoil alongside the diatoms, read our ammonia melt guide, since the two issues often arrive together in week one.
Frequently Asked Questions
Are brown diatoms harmful to fish or shrimp?
No. Diatoms are completely harmless to livestock — fish, shrimp, and snails all ignore them or eat them. They're purely an aesthetic issue and a sign of a young ecosystem, not a health hazard. The only tanks where diatoms coincide with danger are uncycled ones, and there the danger is ammonia, not the brown dust.
How do I get rid of diatoms fast?
There's no instant fix, and chasing one usually backfires. The fastest safe approach: large weekly water changes to dilute silicates, wipe the glass, and add otocinclus or nerite snails once cycled. Expect visible improvement in 2–4 weeks and full resolution by month three. Anything promising overnight results is selling algaecide.
Will more light get rid of brown algae?
Counterintuitively, no — diatoms thrive in low light, so cranking up the light doesn't suppress them. Worse, excess light in an immature tank invites green algae on top of the diatoms. Keep lighting moderate (6–8 hours) during the diatom phase and let plant establishment and silicate depletion do the work.
Do diatoms mean my tank is cycling properly?
They're a common companion of cycling, but not proof of anything — test ammonia, nitrite, and nitrate to know where your cycle stands. Diatoms appear in the same weeks the cycle matures because both are driven by the new-tank environment, but one doesn't guarantee the other. Keep testing; don't assume.
My tap water is high in silicates. What are my options?
First confirm with a silicate test kit — don't guess. If levels are genuinely high (above ~2 ppm), options include blending tap water with RO/DI water, running a silicate-absorbing resin in the filter, or choosing a substrate that doesn't leach silicates. Most municipal water is fine, though; persistent diatoms are more often a stability problem than a water-chemistry problem.
Brown Today, Gone Tomorrow
Diatoms are the hobby's rite of passage — nearly every successful planted tank wore the brown coat for a few weeks. Wipe the glass, change the water, add grazers when cycled, keep the plants growing, and wait. The tanks that beat diatoms fastest aren't the ones with the most products; they're the ones whose keepers understood it was temporary and let the ecosystem mature undisturbed.
Related: our algae identification chart covers the ten most common types.