Brown film on the sand. Brown dust on the rock. Brown snot with bubbles in it. All three get called “the brown stuff” and all three are different organisms with different causes, and one of them will take over your tank if you treat it like the others.
Diatoms — the harmless one
What it looks like: a fine, gritty, rust-brown dusting over sand and rock. It wipes off glass easily and leaves no strings or slime behind.
What it means: silicates plus a new system. Diatoms are the classic new-tank bloom, and they show up in the first weeks and leave on their own once silicate is consumed and the system matures.
What to do: nothing dramatic. Keep up water changes, add grazers when the tank is ready, and wait. If diatoms persist in an established tank, check your source water — that is usually an RO membrane or resin that needs replacing.
Cyanobacteria — the slimy one
What it looks like: a smooth, slick sheet, usually deep red-brown to purple, that peels off in one piece and traps oxygen bubbles on its surface. It has a distinctive smell. It comes off in sheets rather than crumbling.
What it means: low flow plus excess organics. Cyano lives where detritus settles and water does not move. It is a symptom of dead spots and accumulated waste far more than of any single test result.
What to do: siphon it out physically, fix the flow in the spot where it grows, cut back on overfeeding, and improve detritus export. It responds to husbandry. Chemical treatments exist and work, but if you do not fix the flow it returns, so fix the flow first.
Dinoflagellates — the one to take seriously
What it looks like: brown, stringy or snotty growth with long trailing filaments and visible bubbles, often described as looking like mucus or spider webs. The most diagnostic behavior: it largely disappears at night and returns during the day, and it will come back within a day of you siphoning it out.
What it means: usually a tank with nutrients bottomed out. Dinos thrive where nitrate and phosphate have been driven to zero and the competing algae have been starved out of the way. The over-clean tank is dino territory.
What to do: counterintuitively, stop chasing zero. Get nitrate to a few ppm and phosphate to around 0.03–0.10 ppm and hold them there, so that other organisms can compete again. Beyond that: siphon daily, reduce your photoperiod, run UV if you have it (effective against the free-swimming stage of several species), and be patient. Dinos are a weeks-long project, not a weekend one.
One warning: dinoflagellate species differ enormously in what works against them, and some are toxic to fish and inverts. If you have dinos and livestock is suffering, identify the type before choosing a treatment. Guessing here genuinely costs animals.
The one-minute triage
- Gritty dust, wipes away clean → diatoms. Wait it out.
- Slick sheet, peels in one piece, bubbles on top → cyano. Fix flow and export.
- Stringy snot with trailing bubbles, gone at night → dinos. Raise nutrients, siphon daily, be patient.
Get the visual reference
These three are genuinely hard to tell apart from a description, and easy to tell apart from a photograph. Our Algae & Flora ID Guide is a 25-page side-by-side photo reference to the nuisance algae, cyanobacteria and dinoflagellates — what each one indicates about your system and the approach that actually works on it.
It is an instant PDF download, and it is included in the Complete Reef Care Library. You can also grab our free reef water chemistry chart to keep those nutrient targets in front of you.