Controlling Bryopsis Algae in a Reef Tank

Bryopsis is a tenacious feathery green macroalga most grazers refuse to eat. Learn how it spreads, the elevated-magnesium method, when fluconazole is reported to work, and why prevention beats cure.

Bryopsis is a genus of siphonous marine green macroalgae built from single tubular filaments, with a distinctive feathery, fern-like form. It is widely distributed in tropical and subtropical waters and dominant in eutrophic coastal regions. In aquaria it is one of the more dreaded pests: fast-growing, tenacious, and refused by most clean-up crews, it can overtake the rock surface and create low-oxygen conditions that crowd out other life.

How it spreads

Bryopsis usually arrives as a hitchhiker on live rock or coral frags and then takes hold where nutrients allow. It is remarkably regenerative: extruded protoplasm can survive briefly without a cell membrane before reforming into new units, and the alga reproduces both sexually and asexually. That resilience is why simply pulling it off rarely works on its own.

Distinguishing it from hair algae

Bryopsis is sometimes mistaken for ordinary green hair algae, but it has a structured, feather-like branching pattern rather than the loose, thread-like strands of filamentous hair algae. The distinction matters because Bryopsis resists the herbivore grazing and nutrient control that usually tame hair algae.

Control methods

• Manual removal and nutrient control: remove what you can and keep nutrients in a reasonable range, accepting that holdfasts often regrow.
• Elevated magnesium: a long-standing aquarist method uses a specially formulated magnesium supplement to suppress Bryopsis. In one documented case magnesium was raised from about 1,350 ppm to about 1,500 ppm, with a clear turnaround in roughly 10 days and the Bryopsis virtually gone after four weeks.
• Limited grazers: certain tangs, emerald crabs and sea hares may nibble Bryopsis, but grazing alone is usually insufficient.
• Frag and rock quarantine: inspect and quarantine new live rock and frags to avoid introducing Bryopsis in the first place.

Fluconazole: reported, but use caution

Aquarists have reported eliminating Bryopsis with the antifungal fluconazole. In one account, dosing 20 mg per gallon (with carbon and chemical media removed) cleared a Bryopsis strain in about three days, though the same source notes other strains that did not respond at all and that its mechanism in algae is unknown.