Bryopsis

Identification

Bright to dark green feathery fronds: a central stalk with fine branchlets in two rows, like a tiny fern or feather, usually 2-10 cm tufts. Tufts feel wiry or bristly rather than slimy, anchor hard to rock, frag plugs and overflow teeth, and regrow from the base after pulling.

Often confused with

Green hair algae (Derbesia complex) forms soft, unbranched or loosely tangled strands with no feather structure. Bryopsis shows clear fern-like branching, easiest to see in a close photo or under a magnifier. Cladophora turf is coarser and stiffer with tangled mats and no central feather stalk.

What it targets

Live rock, frag plugs, coral bases and sand-bed edges. Mats trap detritus, shade coral tissue at the base, and create low-oxygen pockets under dense growth.

How it spreads

Hitchhikes on live rock, frag plugs and macroalgae. Fragments and the rooted holdfast spread it within the tank, and it can keep growing at nutrient levels that starve out many other algae, which is why pulling alone often fails.

Control (safest first)

Manual removal: pull tufts out by the base, remove badly covered rock for scrubbing outside the tank, and pick out roots and holdfasts with a pick or tweezers. This reduces biomass before any other step and helps every treatment work better. Nutrient control and competition: steady nitrate and phosphate control, good skimming and a healthy refugium slow regrowth. Bryopsis often persists at low readings, so keepers treat this as support, not a cure on its own. Clean-up crew and grazers: urchins, some crabs, mollusks and sea hares are listed as grazers, and real-world reports are mostly disappointing. One keeper tried urchins and a sea hare with no success before treating. Do not buy a clean-up crew expecting it to finish Bryopsis. Fluconazole (aquarium products such as Reef Flux): the treatment keepers report the most success with. Reef Flux directions state 1 capsule (200 mg fluconazole) per 10 gallons, skimmer off for the first 3 days, activated carbon and chemical filtration removed for the treatment, and a minimum 30 percent water change after 21 days; the maker also warns the product kills refugium macroalgae. Bryopsis typically pales at the tips first and melts over 1-3 weeks. A second round is sometimes needed, and some filamentous algae sold as Bryopsis or GHA respond poorly, which points back to identification. Elevated magnesium: older hobby guidance raised magnesium to hinder Bryopsis. Sources note high magnesium can endanger invertebrates, and results vary widely, so this ranks below fluconazole and manual work. Hydrogen peroxide spot treatment outside the tank: keepers report whitening and die-off on removed rock and plugs over a few days. Peroxide can harm corals and inverts on contact, so it belongs on removed pieces only, with a thorough rinse. Flag: use with care, never broadcast-dose casually.

Natural predators

• Urchins
• Sea hare→
• Some mithrax crabs and mollusks

Field notes

A Reef2Reef keeper confirmed Bryopsis plumosa from close photos after clean-up crew attempts with urchins and a sea hare failed, then reported total eradication in just over 7 days on fluconazole, brushing the dying algae off around day 4 onward. In the long Reef2Reef fluconazole thread, a keeper treating what looked like hair algae saw slow results and was advised the filaments lacked the feather pattern and were more likely Derbesia or another hair algae, with pale tips as the first sign fluconazole was acting at all. A maintenance write-up describes the identification moment by touch: the patch felt like a scouring pad rather than slime, and a ripped chunk showed the green feathery structure, after manual removal first and fluconazole second over about two weeks.