Bubble algae

Identification

Smooth, shiny green spheres, single or in tight clusters, from pea size up to about 2-5 cm for the large sailor's eyeball form. Each bubble is a single tough-walled cell anchored by tiny root-like holdfasts. Smaller Valonia forms make grape-like clusters on rockwork, overflow teeth, frag plugs and pump intakes, often in shaded spots and crevices.

Often confused with

Other green algae stay flat, fuzzy or leafy. Bubble algae is the one that forms firm, glossy, hollow-looking spheres you can roll between your fingers. Red bubble algae (Botryocladia) forms reddish translucent spheres on stalks and is a close look-alike handled the same careful way.

What it targets

Rockwork, frag plugs, coral bases and skeleton edges. Dense clusters shade coral tissue, trap detritus, and lift against polyps as they expand.

How it spreads

Arrives as spores or tiny bubbles on frags, plugs and live rock. Popping a bubble in the tank is widely reported to release spores that settle and start new bubbles, so removal happens outside the tank or under a siphon. Available nitrate and phosphate fuel faster growth.

Control (safest first)

Manual removal first: pry each bubble off at the base with a scalpel, fingernail or small screwdriver with pumps off, net or siphon out every piece, and rinse removed rock in tank water outside the display. Staying on top of small patches is the most reliable control keepers report. Nutrient control: keep nitrate and phosphate low and steady with skimming, water changes and a refugium or phosphate media so regrowth slows. Bubble algae can still grow in low-nutrient water, so this supports removal rather than replacing it. Emerald crabs (Mithraculus / Mithrax spp.): the classic biological control and genuinely hit-or-miss. Some individuals clear bubbles steadily, others ignore them or pick at coral. Smaller, younger crabs are reported to work harder on algae. See field notes for both outcomes. Pitho crabs and foxface rabbitfish: pitho crabs are reported working bubbles at night, and larger tanks can use a foxface, which some keepers watched vacuum a covered rock clean. Foxfaces need a large tank and may nip some corals. Vibrant and similar bottled algaecide products: many keepers report bubble algae fading after weeks of dosing, and others report sensitive corals such as Goniopora receding during use. Treat as a last resort, dose conservatively, and watch corals closely. Fluconazole products: some keepers report a double dose stalled bubble algae growth enough to catch up with manual removal. Evidence is thinner than for Bryopsis, so manual removal stays the primary plan.

Natural predators

• Emerald crab→
• Pitho crab
• Foxface rabbitfish→
• Sally Lightfoot crab→

Field notes

Keepers split sharply on emerald crabs: several Reef2Reef posters credit them with clearing every bubble, while others report crabs that ignored the algae entirely, and one keeper removed crabs that went after a Duncan coral instead. One keeper who let bubble algae get ahead described resorting to Vibrant at about double the label rate for roughly a month before it cleared, with no sign of it a year later, paired with much closer inspection of new frag plugs. Another keeper calls bubble algae the easiest algae to remove by hand, using a small screwdriver with pumps off and a net, and notes it comes and goes in waves in a 40-gallon tank without a clear trigger. Pitho crab users in the same discussion reported about half their outbreak gone from night grazing alone.