Diatoms vs Dinoflagellates in a Reef Tank: Visual Diagnosis
Both show up as brown growth on sand, rock and glass. What separates them is how they behave across a single day — dust that sits still, or strands that bubble, lift and come back with the lights.
The short answer
Diatoms are a golden-brown dust that wipes off glass, sand and rock and comes back slowly over days. Dinoflagellates are rust-brown, stringy or sheet-like, commonly hold trapped air bubbles, and typically thin out overnight and return once the lights come back on. Colour alone will mislead you; behaviour over one light cycle separates them far more reliably. If it puffs off as dust and settles again, treat it as diatoms. If it strands, bubbles and disappears before lights-on, treat it as dinoflagellates — and change nothing quickly.
- Dust that wipes offGolden-brown film, no bubbles, returns over several days. Usually diatoms.
- Strands that hold bubblesRust-brown strands or sheets lifting off rock. Usually dinoflagellates.
- Gone before lights-onThinner or missing in the morning, back by mid-morning. The strongest clue you can see.
- Confirm before you actLook at the same spots three times in one day before touching the tank.
Some dinoflagellates produce compounds that are harmful to other tank life, and keeper reports of snail and other invertebrate losses after a heavy bloom are common. Keep your hands out of the water where you can, wear gloves when you must reach in, do not blast the growth into the water column, ventilate the room, and wash up afterwards. This page helps you identify what you are looking at. It is not a treatment protocol.
Compare the two side by side
Nine cues, one table. Work down it with your own tank in front of you rather than from memory.
| Cue | Diatoms | Dinoflagellates |
|---|---|---|
| Colour | Golden brown to light brown, usually even across a surface. | Rust brown to dark brown, often patchy, sometimes reddish under strong light. |
| Texture | Powder or dust. No visible structure. | Strings, sheets, mucus webs, or a film with strands running through it. |
| Bubbles | None. | Common — strands lift off surfaces with trapped air. |
| Overnight behaviour | Looks much the same in the morning as it did at night. | Thinner or gone before lights-on, back by mid-morning. |
| Where it settles | Glass, sand surface, new rock, new sand, low-flow corners. | Rock, sand, coral bases, frag plugs, pump and nozzle bodies. |
| When you wipe or fan it | Comes off as dust and returns over days. | Comes off in strands and sheets and returns within hours. |
| When keepers report it | Often in newer tanks, or after new sand, new rock, or a change in source water. | Often in tanks that have run very low on the nutrients keepers normally test for. |
| Effect on the tank | Mostly a film over surfaces and light. | Coats surfaces, irritates what it settles on, and can spread into the water column. |
| Usual direction | Commonly fades as the tank matures and surfaces are grazed. | Commonly persists or returns until the cause is addressed. |
The three-point check you can run today
Pick two or three spots you can find again — a patch of sand, a piece of rock, one corner of the glass — and look at the same spots three times in one day.
- Before the lights come on — First thing in the morning1. Observe initial stateNote whether the growth is there at all, and exactly where it sits.
- At mid-photoperiod — Halfway through the light period2. Observe active growthNote the colour, whether strands or sheets have formed, and whether bubbles are trapped in it.
- An hour after lights-out — Once the tank is dark3. Observe night transitionNote whether the growth has thinned, lifted, or moved into the water column.
- Any time — Using a turkey baster or powerhead4. Fan one patchDust that puffs off and settles again is one thing. Sheets that peel away and drift are another.
- Same time each day5. Wipe one small area of glassNote how fast that patch returns, and whether it returns as dust or as strands.
- After each of the three looks6. Write down what you sawSame spots, same day, in your own words. Compare the notes before you change anything.
Repeat the three-point check on at least two separate days before you decide. A single look at a single hour is how most of these misidentifications happen, because the two organisms look most alike in the middle of the light period.
Which one are you looking at?
Answer in order. Stop at the first question you answer yes to.
There is brown growth on sand, rock or glass.
1. Does it lift off in strands or sheets, with bubbles trapped in it?
No → Continue to question 2
2. Is it thinner or gone before the lights come on, and back by mid-morning?
No → Continue to question 3
3. Does it wipe off as dust and return over several days?
No → Continue to question 4
4. Is it slimy or mat-like, or green and hair-like rather than brown?
This tree is triage, not identification. The only way to confirm which organism you have is a microscope.
The order that works
Seven steps, in this order. Do not skip to the last one.
Complete the three-point check across at least two days and write down what you saw before you touch anything.
Test and log the parameters you normally track — salinity, alkalinity, nitrate, phosphate and temperature — with your own kits, on the same day. Note the date next to the values. Those numbers decide which step comes next.
Siphon or wipe the patches you can get to during your normal maintenance. Run a fine filter pad or floss in the flow path to catch what lifts off, and change it out. Do not blast growth off rock into open water.
A large water change is not a reliable fix for either problem, and in a tank that already tests low for the nutrients you track it removes what the system is short of. Stay with your normal schedule unless you have a separate reason to change more.
Pick a single thing to adjust — flow, photoperiod, feeding, or filtration media — change only that, and leave it alone long enough to see what it did. Several days of the same three-point check is a reasonable minimum before the next change.
Test at your normal cadence and log the results next to the previous ones, so you are reading a trend instead of a single number.
If the growth holds after the low-risk moves, get the growth identified under a microscope or get a second opinion before using any product. Bring your log and your photos with you.
Mistakes that make it harder
These six are the ones that cost the most time.
The clue that separates these two only exists if you look at the tank at different times of day.
A new-tank diatom film and a dinoflagellate bloom need different responses, and the difference comes from the check.
In a tank that already tests low, this can make the problem harder to solve rather than easier.
Anything strong enough to kill cells also reaches your invertebrates and corals. Do not dose what you have not confirmed.
When you change three variables and the growth recedes, you have learned nothing you can use next time.
It spreads to new surfaces, and it puts whatever it contains into the water you are leaning over.
Edge cases worth knowing
Six situations where the standard check needs a caveat.
Both nutrients read zero
It is only on the sand, and it vanishes every night
It is growing on the corals
You have two things growing at once
Animals look wrong, or something has died
You are considering a lights-out period
Questions we get about brown growth
The six we are asked most often.
Can I tell diatoms from dinoflagellates without a microscope?
Do dinoflagellates go away on their own?
Are dinoflagellates dangerous to fish and corals?
Will a big water change fix it?
Should I test for silica?
Should I use a product that says it kills dinoflagellates?
Where to go next
Related reading on this site, plus the one place most keepers go after a diagnosis.
What a rising reading looks like on the glass, the sand and the rock, and how to confirm it before you act on it.
Log Reef Tank Parameters SafelyA conservative order for testing and recording your own numbers, so a diagnosis rests on your tank rather than on a memory.
Corals & InvertebratesWhat the livestock in the path of a bloom actually needs from you while you work the problem out.
Everything on this site starts from the animals you actually keep.
Browse Species Guides