Brown Algae (Diatoms) in a New Tank: Normal or a Problem?
A dusty brown film coating the glass, substrate, and plant leaves a few weeks into a new tank is one of the most predictable events in the hobby. It is diatoms, it is almost always temporary, and the correct response is usually to do very little.
Identifying it
Diatoms form a soft brown or rust-coloured dust rather than a firm growth. The test is simple: wipe the glass with a finger and it comes away instantly, clouding the water. Green algae resists; diatoms do not.
It coats everything evenly — glass, substrate, decor, leaves — rather than favouring high-light areas the way green algae does.
Why it happens in new tanks
Diatoms are single-celled algae that build silica shells. They need silicates, and a new tank has plenty: leaching from new substrate, from tap water, sometimes from the silicone or glass itself.
They also thrive when there is no competition. In a new tank the plants have not rooted in, the bacterial colony is immature, and nutrients are unbalanced. Diatoms are the fastest coloniser of that empty space.
The characteristic timing is weeks two to eight. It appears, spreads over everything, and then — as silicates deplete and plants and green algae outcompete it — it recedes and does not come back.
What to do
Mostly nothing
The default answer is patience. It resolves on its own in most tanks. Wipe the glass at water changes so you can see in, and let the rest run its course.
Manual removal
It comes off trivially. Wipe glass with a sponge or magnet, gently wipe plant leaves with fingers, and siphon it off the substrate during water changes. Do not scrub plants hard — the algae is harmless and the leaf damage is not.
Otocinclus — the actual specialists
Otocinclus catfish eat diatoms enthusiastically and are one of the few algae eaters that genuinely does the job advertised. Two caveats: they need an established tank, so do not add them the week the brown algae appears, and once the diatoms are gone you must feed them — blanched vegetables or algae wafers. Otos starving to death after clearing a tank is common and avoidable.
Nerite snails also graze diatoms effectively and have no such problem.
If it will not go away
Persistent diatoms in an established tank mean an ongoing silicate source, usually the tap water. Options: RO or RO/DI water, silicate-removing resin in the filter, or increasing plant mass to outcompete it.
Also check that light is adequate. Diatoms tolerate low light better than most plants, so a tank that is too dim for good plant growth tends to favour them.
What not to do
- Do not use algaecides. Overkill for a temporary problem, and hard on plants and invertebrates.
- Do not blackout the tank. Effective against green water; diatoms tolerate low light fine, so you stress your plants for nothing.
- Do not do enormous water changes to fix it. If your tap water is the silicate source, you are adding fuel.
- Do not add fish to eat it immediately. A new tank is not ready for otos.
The reassuring part
Brown diatom algae is harmless to fish, invertebrates, and plants. It is unattractive and that is the whole problem. In a new tank it is close to a rite of passage, and the keepers who wait it out end up in the same place as the keepers who fight it — just with less effort and fewer chemicals.
If it appears months into an established tank, that is different: something changed. Check your source water and your lighting.
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