Dinoflagellates in a reef tank: how to identify and get rid of dinos

What dinoflagellates are

Dinoflagellates are single-celled algae. Each cell has two flagella: one lies in a groove running toward the back of the cell, and a flattened, ribbon-like one sits in a groove that wraps around it. Together they "may cause the dinoflagellate to slowly turn on its axis," which gives the group its name, "whirling flagella" (UC Museum of Paleontology). Most are armoured in cellulose plates; some are naked.

The pest species in reef tanks are mostly benthic: they grow on rock, sand and algae, though BRS notes some break apart into the water at night. Many benthic dinoflagellates, including Ostreopsis, Prorocentrum and Coolia, "are known to produce copious amounts of mucilage both in situ and in culture," and Ostreopsis can form floating clusters at the sea surface. That mucus is the snot you see on the rocks.

Dinos, cyano or diatoms?

These three are the usual brown-to-red films in a reef tank, and the treatments differ, so identify before you treat.

• Dinoflagellates: brown or golden strands and mats with trapped bubbles, which Bulk Reef Supply calls "bubbly snot." Stringy when pulled, and they come back quickly after removal. BRS notes some types "reduce or disappear at night and then come back when the lights turn on." The tank often tests zero for nitrate and phosphate.
• Cyanobacteria: a red, pink or black mat that sheets across sand or rock, slimy, and releases small bubbles when stirred. It favours low-flow spots.
• Diatoms: brown dust on glass, sand and rock that wipes off easily, most common in tanks under about three months old. Diatoms build their shells from silica, so silicate in the water feeds them.

No microscope? One hobbyist method from Reef Central is a bottle test: siphon a dense sample into a capped bottle, shake it hard, pour it through a paper towel or filter sock, and leave the filtered water in room light for 30 to 120 minutes. Its author says dinos are the only type that "aggregates back together quickly in the container," while cyano stays matted in the filter and diatoms settle. It is a forum method, not a validated test, so use a microscope if you can.

Microscope ID: which dino is it?

Treatment results depend on which dinoflagellate you have: BRS says UV only helps types with a free-floating stage, and toxicity differs by genus and strain. Take a sample of the film with a pipette, put a drop on a slide, and look at it while the cells are alive. The genera below range from roughly 10 to 100 µm long, so a basic compound microscope at 100–400× shows them.

First, the group:

• Dinoflagellates: single, often golden-brown cells, swimming with a turning or spinning motion from their two flagella.
• Diatoms: cells inside a hard, porous shell, the frustule, "composed almost purely of silica," so they look rigid and glassy.
• Cyanobacteria: bacteria with no nucleus, which "grow in colonies or filaments"; Oscillatoria filaments sway under the microscope (UC Museum of Paleontology).

Then the genus. Sizes come from published measurements; the movement notes are from a hobby ID guide published by Pod Your Reef, a copepod retailer:

• Ostreopsis: ovoid to teardrop-shaped. O. cf. ovata cells measured 42–105 µm long in a Portuguese bloom; a Gibraltar strain of another Ostreopsis species measured 38–59 µm. The hobby guide says anchored cells "spin in place 'like a tetherball'."
• Coolia: "small, compressed, rounded and lens-shaped" with a golden-brown chloroplast, 30–38 µm long (C. monotis).
• Prorocentrum: P. lima cells are oblong to oval, widest in the middle and narrower at the front, 37–48 µm long. The hobby guide describes slower, wobbling movement than Amphidinium.
• Amphidinium: the small one. A. carterae is under 20 µm long, oval or tongue-shaped, with no armour plates, and is a benthic species "common in association with sandy beaches." The hobby guide says cells "slide and dart smoothly across surfaces" and stay on the sand.

Which dinoflagellates are toxic

The genera hobby guides most often report in reef tanks include toxic species. What is known comes from wild and lab strains; we found no published measurement of toxins in a home aquarium outbreak.

• Ostreopsis: the main concern. O. cf. ovata "can produce palytoxin (PLTX), ovatoxins (OVTXs) and mascarenotoxins", and a southern Mediterranean strain contained ovatoxin-a and -b. In lab tests on crustaceans and young sea bass, brine shrimp were the most sensitive, affected even at concentrations below the environmental alarm threshold, and the toxicity depended on living cells. Ostreopsis blooms have also been linked to respiratory symptoms and malaise in people breathing sea-spray aerosols.
• Prorocentrum lima: makes okadaic acid and dinophysistoxin-1 in culture, and has been associated with diarrhetic shellfish poisoning.
• Amphidinium: produces amphidinols, which a 2023 study describes as having "hemolytic, cytotoxic, and fish mortality properties." At higher concentrations, A. carterae raised mortality and malformations in zebrafish embryos.
• Coolia monotis: "seems to be non-toxic or to include toxic and non-toxic strains"; a Gibraltar strain produced no detectable cooliatoxins.

Toxin output varies by strain. The Gibraltar Ostreopsis strain produced no detectable ovatoxins or palytoxins, while the southern Mediterranean O. cf. ovata strain did. A microscope can tell you the genus, not whether your outbreak is producing toxins, so treat every outbreak as potentially toxic.

Why low nutrients are linked to dinos

The link is strong in hobby experience and unproven in the lab. Bulk Reef Supply: "While not proven, repeated anecdotal evidence points to zero nutrients in the water either cause or trigger dinos," and "it is the double zero we are really looking out for": nitrate and phosphate both at zero. BRS adds that dinos may outcompete other organisms when nutrients run that low, and that carbon dosing and biopellets may contribute. Carbon dosing can run phosphate to zero while nitrate is still measurable (see vinegar and vodka dosing).

One possible reason is mixotrophy: some dinoflagellates photosynthesise but can also eat. In a lab study of one planktonic species, feeding boosted photosynthesis "particularly when the cells were grown under low light and/or nutrient-limited conditions," and growth was 2 to 3 times faster than on photosynthesis alone. Nobody has tested whether that explains outbreaks of the benthic pest genera in aquariums, so treat it as a hypothesis.

How to get rid of dinos

Bulk Reef Supply's protocol has four parts: raise nutrients, add bacteria, remove the dinos physically, and black the tank out. UV is an add-on for the right type. This matches the short version on our algae identification page.

1. Stop stripping nutrients

Stop carbon dosing and biopellets (BRS). Take phosphate removers such as GFO offline too, since you are about to try to raise phosphate. BRS also suggests reducing filtration efficiency.

2. Raise nitrate and phosphate, slowly

The minimum target, in BRS's words: "make adjustments until you are maintaining levels above zero and show up on your home water test kit." Then work toward the reef ranges on our parameter pages, nitrate 2–10 ppm and phosphate 0.02–0.10 ppm. A colour-card kit is hard to read near zero; the phosphate page covers the low-range checkers that can tell 0.00 from 0.02 ppm.

BRS gives two ways: feed more, or dose nitrate and phosphate, naming Brightwell NeoNitro and NeoPhos. Both labels are written for ultra-low-nutrient systems: NeoNitro's first target is about 3 ppm nitrate, then 3–5 ppm, and NeoPhos's about 0.02 ppm phosphate. Dose from the label on your bottle, not a number from a forum or an old article. NeoPhos has been sold at more than one strength (one Reef2Reef user found the new bottle about 24 times weaker than his old one), and the NeoNitro label tells you to go by the gallons treated on the bottle. Add a little, retest the next day, and don't overshoot. Above 0.10 ppm, phosphate starts feeding other nuisance algae.

3. Siphon it out every day

BRS says to remove growth as often as possible, "preferably everyday until the battle is won." Siphon the colonies off rock and sand into a bucket and throw that water away. "Stirring it up does not have the same effect." For types that break apart into the water at night, change the filter sock every morning; BRS also suggests a roller mat or a reactor with a 5-micron sediment filter.

4. Dose bacteria

BRS suggests heterotrophic bacteria products such as Dr. Tim's Waste-Away or Brightwell MicroBacter7, possibly as a mix, though it notes there is no agreed recipe. Dose them at the label rate.

5. Black out for 3–4 days

Dinos are photosynthetic. BRS's blackout is lights off and ambient light blocked for "3-4 consecutive days," with an opaque cover over the tank. Do it after nutrients are measurable and the daily siphoning has started, not instead of them.

6. UV, for the right type

BRS: "UV sterilizers can only kill what passes through them," so dinos that stay on the rock or sand are untouched. The types that vanish at night and return by day do have a free-floating stage, BRS says, and UV can reach them. Even then, "there are no guarantees." BRS's example is a 25 W Pentair unit on a tank of up to 130 gallons at the faster algae-and-bacteria flow; our UV sizing guide has the manufacturers' flow ratings. The hobby ID guide puts Ostreopsis and Prorocentrum in the free-floating group and Amphidinium on the sand, where UV does little.

Timeline: BRS warns of "weeks, even months of persistence," with progress visible within weeks and the outbreak gone within a month or two.

Silicate and diatoms

Some reefers dose silicate to start a diatom bloom, on the theory that diatoms will outcompete the dinos. BRS describes hobbyists "experimenting with silicate dosing to purposefully spark diatom growth" to "out-compete those dreaded dinoflagellates," and treats it as an experiment, not a recommendation. We found no controlled test, no published target level, and no product labelled for this use, so we don't give a dose. If you try it, use a product as its label directs, measure silicate with a test kit, and expect brown diatom dust; that is the point.

Peroxide, Dino X and other products

Fauna Marin Dino X is the one product we found with published dinoflagellate directions. BRS lists it as discontinued; if you have a bottle, follow its label:

• 5 mL per 100 L (26 gal) of total system water, up to 6 mL per 100 L for heavy outbreaks, every 2 days. Dinoflagellates "can usually be treated with 10 doses." "DO NOT OVERDOSE": measure your real water volume, allowing for rock and sand.
• Dose in the evening or an hour after the main lights go off, and keep the main lighting period to 6 hours a day or less.
• Run the skimmer wet, 24/7. No carbon or ozone during treatment; GFO and phosphate removers off.
• No water changes during treatment; resume them a week after the last dose. No trace elements, amino acids or SPS foods during treatment.
• Run fresh activated carbon afterwards to remove residual chemicals. BRS staff add that it affects all algae, so take macroalgae and refugiums offline first.

Hydrogen peroxide has no label for dosing into a reef tank. The doses that circulate come from uncited retailer and forum guides, and even a retailer that promotes it warns of polyp retraction in corals and that for hermit crabs "Prolonged exposure can be lethal." We don't give a dose and don't recommend dosing it into the display.

Antibiotics and copper aren't labelled for dinoflagellates in reef tanks, and general algaecides don't name them. Don't use them.

Keeping fish, corals and yourself safe

• Assume it is toxic. Strains differ, and the toxic genera are among those hobby guides report in tanks. Wear gloves when you siphon or handle affected rock, wash up afterwards, and avoid splashing or spraying tank water, given the link between Ostreopsis blooms and respiratory symptoms from aerosols.
• Get it out, don't spread it. Siphon colonies to waste and change socks daily rather than blowing them around the tank.
• Raise nutrients within the ranges. Fish tolerate far more nitrate than corals do, so the targets above don't put fish at risk. Overshooting phosphate costs you other algae and slower coral calcification (see the phosphate page).
• Follow product labels exactly, including what to switch off, and run carbon after a chemical treatment ends if the label says to.

What not to do

• Don't cut nutrients further. Don't chase dinos with big water changes or stronger export: salt water mixed from RO/DI has little or no nitrate and phosphate, so it dilutes what you are trying to raise. Stop carbon dosing and GFO instead.
• Don't stir the dinos up instead of siphoning them out (BRS).
• Don't expect UV to fix types that stay on the sand or rock.
• Don't copy a NeoPhos or NeoNitro dose from a forum post. Dose from your bottle's label and test.
• Don't dose peroxide, antibiotics, copper or unlabelled algaecides into the display.
• Don't assume your dinos are harmless because of their genus.

For other nuisance algae, see the algae identification page and the bubble algae guide; for the nutrient background, see ideal reef parameters and the ULNS glossary entry.