Diatoms are a brown or golden, dusty film on sand, rock and glass. Almost every new reef tank gets them in its first few months. They usually fade on their own within a few weeks. Keep wiping and siphoning, check your RODI water for silicate, and add a few grazing snails.
How to recognise them
Manta Systems describes diatoms as single-celled algae with silica cell walls. They form a thin brown dust that wipes off the glass easily and blows off rock with a turkey baster. Unlike cyano or dinoflagellates, they don’t form slimy sheets, stringy strands or trapped bubbles.
Why new tanks get them
A new tank has few organisms competing for nutrients, and new rock and sand can release silicate, which diatoms use for their shells. Manta Systems says almost every new reef sees a bloom in its first few months, typically lasting 2–6 weeks.
- New dry rock and sand releasing silicate as they cure
- Tap water, or RODI water with exhausted DI resin
- Overfeeding or immature filtration
- Small amounts in some salt mixes
What to do
Mostly, be patient and keep the tank clean while the bloom runs its course.
- Wipe the glass and siphon the sand surface every few days
- Test your RODI output with a TDS meter and replace the DI resin if it’s exhausted. See TDS and source water.
- Add a small clean-up crew once the tank has cycled: Manta Systems lists trochus, cerith and nerite snails
- Keep phosphate low and filter socks clean
- Increase flow in dead spots where it collects
When to worry
Manta Systems suggests looking harder if diatoms last beyond about two months: test your RODI water and check for low-flow areas. Watch out for one trap: big water changes with silicate-containing water can feed a bloom rather than clear it.
If the film turns slimy, stringy or red, it may be something else; compare with the cyano and dino guides.
Common questions
Are diatoms harmful?
Generally no. They’re unsightly but a normal stage in a new tank.
How long do diatoms last?
Manta Systems gives 2–6 weeks for a typical new-tank bloom.
Should I add more snails?
A few grazers help once the tank has cycled. Don’t add more than the tank can feed once the diatoms are gone.
Sources and limits
Last checked 12 October 2026. This guide brings together the published advice below. It can’t see your tank: if you’re unsure what you’re looking at, a clear photo to an experienced local shop or club helps.
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