Dinoflagellates ("dinos") are one of the trickiest nuisance organisms in saltwater aquariums. They typically show up as brown, slimy coatings on sand and rock, sometimes with bubbles trapped in the slime. They look like algae, but they're actually microscopic protists. Some species are harmless. Others can wreak havoc on your reef.
Common types of dinoflagellates
Identifying the type can help inform your approach. The most common ones aquarists run into:
- Ostreopsis. Toxic, sticky, and dangerous to snails and corals. Forms dense mats and traps bubbles.
- Amphidinium. Free-floating and persistent. Often survives even through blackouts.
- Prorocentrum. Smaller and less aggressive. May appear as a light dust on surfaces.
- Coolia. Attaches to glass and sand. Moderately toxic and relatively slow-growing.
- Gambierdiscus. Rare but highly toxic. More common in tropical environments.
- Symbiodinium. A natural coral symbiont (zooxanthellae). Not considered harmful in this context.
How to identify them
- Appearance. Slimy or dusty brown layers, often with trapped bubbles.
- Timing. Often intensifies during light periods and clears overnight.
- Behaviour. No visible movement, but the patches may shift location daily.
- Impact. Irritated corals, dying snails or inverts, and ultra-clear water from microfauna die-off.
Why dinoflagellates appear
Most outbreaks come from an imbalance. Common causes:
- Ultra-low nutrient levels (undetectable nitrate or phosphate)
- Overuse of UV or aggressive mechanical filtration
- Recent tank resets, sterilisations, or bacterial blooms
- Minimal biodiversity (new tanks, or systems stripped after pest treatment)
Effective treatment steps
There's no one-size-fits-all cure, but this multi-step approach is widely successful.
1. Identify the species (optional)
A microscope helps distinguish between strains, especially Ostreopsis vs Amphidinium. Not mandatory, but it improves targeting.
2. Blackout the tank (2 to 3 days)
Dinoflagellates rely on photosynthesis. Blocking all light, including ambient room light, interrupts their energy cycle and weakens them significantly.
3. Stabilise nutrients
Raise nitrate to 5 to 10 ppm and phosphate to 0.05 to 0.1 ppm. Dinos thrive in ultra-clean tanks. Rebalancing the nutrients often slows or halts their growth on its own.
4. Dose a targeted treatment (optional)
Products like Dino Destroy can disrupt dinoflagellate cell function without harming corals or fish. Most effective when combined with the other steps above.
5. Lean on the community
We recommend joining Mack's Reef Dinoflagellate Support Group on Facebook. They can help ID the type of dinos you have and offer advice on outcompeting them. The most common community-tested approach is seeding the tank with diatoms, which fight dinos for the same resources.
Prevention
- Don't drive nutrient levels to undetectable
- Avoid excessive sterilisation or filtration
- Introduce biodiversity slowly and deliberately
- Watch for changes after major tank cleanouts or new additions
- Quarantine new rock, coral, or sand that may carry pests
The bottom line
Dinoflagellates are frustrating but beatable. Success comes from understanding what triggers them and adjusting your system to support a healthier, more balanced microbial environment. Stay patient, stay consistent, and use a targeted product to tip the scales when needed.
