Brown diatoms in a new tank
Your tank is a few weeks old, and a dusty brown film is gradually creeping over the sand, the glass and the plants. It looks like rust or dust. Those are diatoms — brown algae — and they’re an almost unavoidable stage of startup. Good news: they go away on their own.
The essentials. Diatoms (a brown film) are harmless algae, typical of a tank under 2 months old. They feed on silicates that are abundant in a new tank. They disappear on their own in 2 to 8 weeks as the tank matures. You can speed things up by wiping the glass and adding grazers (otocinclus, nerite snails), but the real fix is patience.
Why this brown film appears
A new tank brings together perfect conditions for diatoms:
- water and materials (sand, glass) rich in silicates, their food;
- a competing flora (good bacteria, established plants) still absent;
- the light already switched on, but an ecosystem not yet balanced.
Diatoms are simply colonising empty space. As soon as the tank matures — the silicates run out, plants and bacteria take over — they have nothing left to live on and recede.
Diatoms are almost a good sign: they show that your tank is in full maturation. Many beginners mistake them for a failure and tear everything down. That's pointless: they're part of the normal process.
Should you do anything?
Not really — but you can lend a hand:
- I wipe the glass (cloth/scraper) for the sake of looks
- I lightly siphon the sand during water changes
- I don't over-light (6-8 hrs/day is enough)
- If the tank is cycled: I consider otocinclus or nerite snails (diatom grazers)
- I'm patient: they recede as the tank matures
⚠️ Only introduce diatom grazers (such as otocinclus) into a cycled and somewhat mature tank: these delicate fish need an established biofilm to feed on. Putting them in a tank that’s too new is effectively starving them.
Diatoms or another algae?
- Dusty brown, wipes off easily → diatoms (what we’re describing here).
- Green on the glass → green algae (excess light).
- Green strands → hair algae (see the dedicated article).
- The water itself is green → free-floating algae (see “green water”).
Conclusion
The brown film of a new tank is a classic with no real downside: diatoms exploit a still-immature ecosystem and fade as soon as it stabilises, within a few weeks. Wipe for the sake of looks, optionally help with suitable grazers once the tank has matured, but above all: let time do its work.
For other algae, see “How to get rid of green hair algae”. And for the full overview, our guide “Cloudy aquarium water: full guide”.
Frequently asked questions
Are diatoms dangerous for my tank?
No, diatoms (brown algae) are completely harmless to fish and plants. They're unsightly but benign, and characteristic of the maturing phase of a new tank.
How long do diatoms last in a new tank?
Usually 2 to 8 weeks. They appear in the first month and disappear on their own as the tank matures and the silicates they feed on run out. Patience is the best strategy.
How do I remove diatoms quickly?
Wipe the glass, siphon the substrate, and you can introduce natural grazers like otocinclus or nerite snails. But the key is still to wait: until the tank has matured, they can come back.
Why only in a new tank?
Because a new tank is rich in silicates (from the sand, the water, the glass) that diatoms feed on, and the competing flora isn't established yet. Once the tank matures, those conditions vanish.