Dinoflagellates are single-celled microalgae that can quickly invade a reef aquarium. They often form a brown, sometimes bubbly film, covering live rock, sand, and even corals. Besides their unsightly appearance, some dinoflagellates are toxic to invertebrates and can weaken the balance of your tank.

In this article, Atelier Récifal explains how to recognize a dinoflagellate invasion and what solutions to implement to effectively eliminate them.

1. Recognizing Dinoflagellates

• Appearance: viscous brown film with air bubbles on the surface.
• Location: on sand, rocks, and sometimes glass.
• Behavior: they often disappear at night and quickly reappear with light.

💡 Be careful not to confuse them with simple brown algae (diatoms).

2. Understanding the Causes

Dinoflagellates generally appear following an imbalance:

• Nutrients too low (NO₃ and PO₄ close to zero).
• Excessive light or unsuitable spectrum.
• New water without competing microfauna.
• Overuse of anti-phosphate resins.

👉 A “too clean” tank often promotes their appearance.

3. Solutions to Eliminate Them

🔹 Adjust Nutrients

• Maintain measurable levels of NO₃ (2-5 ppm) and PO₄ (0.05-0.1 ppm).
• Avoid aiming for “zero nutrients,” which weakens bacterial and algal competition.

🔹 Reduce Light

• Decrease lighting intensity and duration for a few days.
• Check the spectrum: favor blues and reduce whites.

🔹 Strengthen Biological Competition

• Introduce or boost microfauna (rotifers, copepods).
• Use live bacteria to enhance biodiversity.
• Some detritivores (snails, sea urchins) can help limit their spread.

🔹 Additional Methods

• UV filtration: effective against free-floating forms in the water.
• Efficient skimmer to eliminate toxins.
• Regularly siphon affected areas.

⚠️ Avoid miracle chemicals: they mask the problem without addressing the cause.

4. Preventing Their Return

• Maintain a balance of nutrients / light / flow.
• Do not over-sterilize tank water.
• Regularly monitor your parameters.

💡 Atelier Récifal Tip: a balanced reef aquarium with rich microfauna is the best weapon against dinoflagellates.

Conclusion

Dinoflagellates are a nightmare for many reef keepers, but with a good understanding of their origin and a comprehensive approach (nutrients, light, biodiversity), it is possible to eliminate them permanently.

At Atelier Récifal, we support enthusiasts in managing and succeeding with their reef aquariums.