algae · saltwater tank

Bryopsis (feather algae)

Bryopsis is a tough fern-like macroalgae that's notoriously hard to remove. Once established it dominates rockwork and resists most CUC species.

Reviewed by Fast Aquatics husbandry team · Updated May 2026
Severity: Moderate - cosmetic + competitive with coral

Symptoms

What causes it

Hitchhiked in on rock or coral; sustained phosphate or iron excess fuels growth.

Treatment options

Always treat in a separate quarantine tank.

Magnesium elevation (Tech-M). Raise magnesium to 1600-1800 ppm (using Kent Marine Tech-M specifically) for 4-6 weeks. Bryopsis dies; most coral tolerates the elevated Mg. Drop back to normal range after.
Fluconazole (Reef-Flux). Effective for severe infestations: dose per protocol for 14-21 days. Filter heavily for resulting die-off.
Manual removal. Limited effectiveness alone - bryopsis regrows from cells left behind.

Prevention

Dip every new piece. Quarantine new rock. Monitor magnesium for accidentally low levels.

Frequently asked questions

What does Bryopsis (feather algae) look like?

Fern-shaped feather-like green growth. Bright green color. Resists most snails + crabs.

What causes Bryopsis (feather algae)?

Hitchhiked in on rock or coral; sustained phosphate or iron excess fuels growth.

How is Bryopsis (feather algae) treated?

Magnesium elevation (Tech-M): Raise magnesium to 1600-1800 ppm (using Kent Marine Tech-M specifically) for 4-6 weeks. Bryopsis dies; most coral tolerates the elevated Mg. Drop back to normal range after.

Can Bryopsis (feather algae) be prevented?

Dip every new piece. Quarantine new rock. Monitor magnesium for accidentally low levels.

How fatal is Bryopsis (feather algae)?

Moderate - cosmetic + competitive with coral

Related

Browse the full disease database for 45 aquarium conditions with treatment protocols, or check the care library for prevention-focused husbandry guides. Use our symptom matcher to rank likely diseases from observed signs, the water parameter checker to diagnose related water-quality issues, or the QT timeline calculator to plan a treatment schedule.