Cybergerac (CC BY-SA 4.0)
Dinoflagellates (problem dinos)
Ostreopsis spp., Prorocentrum spp., Amphidinium spp. (large-cell and small-cell) and Coolia spp.
Toxic types (notably Ostreopsis) can kill snails and stress fish and corals, and blooms coat sand and rock fast in low-nutrient tanks.
Identification
Rusty brown to golden, snotty or stringy coating on sand, rock and coral, often with small oxygen bubbles trapped in it by late in the light period. Strings can lift off surfaces like mucus. Patches often look lighter or seem to fade overnight and return strongly under lights. A microscope view is the reliable way to name the genus, and the genus decides the treatment.
Often confused with
The classic three-way mix-up: cyanobacteria forms smooth slime mats that peel in sheets and can be red, green or purple; diatoms form a dusty brown film with no strings or bubbles and feel gritty; dinoflagellates are stringy and snotty with bubbles and a mucus texture. A cheap microscope settles the ID, and keepers treat ID as step one.
What it targets
Sandbed, rockwork, macroalgae and coral surfaces. Toxic strains poison grazing snails and hermits first, can irritate fish, and mucus mats shade and stress corals, with tissue loss reported in heavy blooms.
How it spreads
Present at background levels in most tanks and blooms when conditions favor them: nitrate and phosphate held at or near zero for a stretch, dry-rock starts with little microbial competition, and sterile, ultra-clean systems are the settings keepers report most often. Cells spread through the water column at night for some genera, while Amphidinium stays in the sand.
Control (safest first)
Identify the genus under a microscope before buying gear: UV, silicate dosing and nutrient plans differ by genus, and treating the wrong type wastes weeks. Raise and hold nutrients: keepers report success holding roughly 10 ppm nitrate and 0.1 ppm phosphate, tested daily at first, so green algae, diatoms and other microbes can compete again. This is the shared first step across most success stories. Run activated carbon and change it often during toxic blooms: carbon is reported to pull dino toxins and protect snails and other livestock while the bloom is worked down. UV sterilizer, correctly sized with slow flow, plumbed to draw from the display: very effective on Ostreopsis, which enters the water column at night, and helpful on Prorocentrum. UV does little for Amphidinium, which stays in the sand, so genus ID matters. Siphon and filter: blow mats loose and filter water through a fine (about 1 micron) sock, returning the filtered water rather than doing constant large water changes, which can drop nutrients further. Silicate dosing to grow competing diatoms: some keepers deliberately dose silicates so a diatom bloom outcompetes Amphidinium in the sand, then let the diatoms fade. Expect an ugly brown phase during this plan. Increase biodiversity: live rock, copepods, phytoplankton and bacterial diversity are reported to help the system resist a repeat bloom once nutrients are stable. Blackouts alone rarely finish dinos and they often rebound within hours to days of lights returning; keepers use short blackouts only as support for UV and nutrient work. Chemical dino products get mixed reports and rank last.
Natural predators
No natural predators are listed.
Field notes
A Canreef keeper traced a 75-gallon bloom to nitrate and phosphate sitting at zero, identified Prorocentrum plus Amphidinium under a borrowed microscope, and reported the rock clearing in about 3 days with an in-tank UV unit, alongside carbon changed frequently for toxins. Reef2Reef advice repeats one warning in several threads: UV can wipe out Ostreopsis within days and does nothing for Amphidinium in the sand, so the microscope ID comes before the UV purchase. Another keeper beat dinos with silicate dosing to fuel a competing diatom bloom plus bacterial dosing and a short lights-out period, after nitrate rising on its own failed to slow the dinos down.