Bubble algae (Valonia) is a shiny, bottle-green single-celled alga that grows in clumps on rock. Remove it by hand with the pumps off, add emerald crabs to graze it, and keep checking every week. Reef Builders says popping the bubbles doesn’t spread it, so you don’t need to worry about breaking them.
What it is and where it comes from
Reef Builders calls Valonia one of the most persistent pests of mature reef tanks. It usually arrives on corals and rock. Left alone, it can overgrow corals, block overflows and clog wavemaker intakes.
Each bubble is a single cell, among the largest single-celled organisms known.
Removing it by hand
Turn the wavemakers off first, so loose bubbles don’t end up stuck to a pump cage. Then lever the bubbles off with your fingers, tweezers or a metal scraper. Siphon or net any that float off, and scrub the remains off the rock with an old toothbrush.
Reef Builders says popping the bubbles doesn’t release spores, despite what you may have read elsewhere. Getting the base off the rock matters more than keeping each bubble whole.
Animals that eat it
Emerald crabs (Mithrax sculptus) are the usual choice. Reef Builders says they work best when they aren’t overfed by fish food, and warns that some turn on zoanthids or grab at fish. In larger tanks, a foxface rabbitfish can help, but it needs a tank of around 1.5 m and 380 L or more.
Choose animals that suit your tank as adults, and add them in quarantine first if you can.
Keep checking
A few bubbles become a carpet over months. A quick weekly look, with anything new picked off straight away, is far easier than a big clean-up later. Note new corals and rock in your Companion notes, so you can see where an outbreak started.
Common questions
Does popping bubble algae spread it?
Reef Builders says it doesn’t release spores. Remove the base from the rock so it doesn’t regrow.
How many emerald crabs do I need?
It depends on the tank and the outbreak. Start with one or two, and avoid overfeeding so they eat the algae.
Will bubble algae harm corals?
It can grow over them and shade them. Remove it from around corals first.
Sources and limits
Last checked 13 October 2026. This guide brings together the published advice below. It can’t see your tank: if livestock is in distress, act first, then ask an experienced local shop or club, ideally with your test log to hand.
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