Bubble algae (Valonia) in a reef tank: how to get rid of it

What bubble algae is

The bubble algae most reefers fight is Valonia ventricosa, also published as Ventricaria ventricosa; Ventricaria is now treated as a synonym of Valonia. The Smithsonian calls it one of the biggest single-celled organisms known: each bubble is one cell, up to 5 cm across. It grows attached to hard surfaces by small rhizoids, the Smithsonian notes it tolerates low light, and AlgaeBarn says it settles into protected cracks, crevices and shaded spaces. Related species such as Valonia utricularis grow as clusters of smaller bubbles.

A few bubbles do no harm. The problem is spread: Bulk Reef Supply says that left unchecked it can cover live rock and corals, and Reef Builders warns it can block overflow weirs and wavemaker flow.

Bubble algae or something else?

Several things in a reef tank look like bubbles. How to tell them apart:

• Valonia (bubble algae): AlgaeBarn's description is "a perfectly round, shiny green marble." Grass green to dark green, smooth, single or in clusters, each bubble held to the rock by small rhizoids rather than a stalk.
• Dictyosphaeria (also called green bubble algae): a saclike, hollow body that is "spherical when young, becoming convoluted, ruptured, and irregularly lobed when old," built from "large bubble-shaped cells that are easily seen by eye" (University of Hawaiʻi). It reads as a lumpy green crust of small cells, not one marble.
• Red grape algae (Botryocladia): red to burgundy, with cylindrical branches that end in "smooth, shiny bubbles" in grape-like clusters (Meerwasser-Lexikon). The colour and the stalks give it away.
• Grape caulerpa (Caulerpa racemosa): runners anchored by rhizoids, with upright branches covered in "spherical or ovate side-shoots" (Wikipedia). The beads sit on stems, not on the rock.
• Gas bubbles in brown or red slime: not algae cells at all. Bubbles trapped in brown snot point to dinoflagellates; a red, pink or black mat that releases bubbles when stirred is usually cyanobacteria. The algae identification page covers both.

How it gets in and why it spreads

Bubble algae doesn't appear from nowhere. Bulk Reef Supply says it will "often hitchhike into aquariums via live rock, corals, or other substrates", usually as a single bubble on a frag plug or a tiny bubble in transferred water, and sometimes on snail and crab shells. From there, "Even a single, unnoticed cell can establish and proliferate under suitable conditions." BRS adds that "Bubble algae can thrive even in tanks with good water quality," so it isn't proof that your parameters are off.

It spreads two ways. Mature Valonia cells reproduce by swimming zoospores; in V. utricularis and V. fastigiata, researchers recorded three types. It also spreads vegetatively from its attachment structures, so a rhizoid left on the rock can regrow (AlgaeBarn). That is why BRS says to remove "the entire structure to prevent regrowth." The cheapest control is at the door: look over every frag plug and rock before it goes in, and pick off any bubble you find.

Does popping bubble algae spread it?

The standard forum warning is that a popped bubble showers the tank with spores. Two hobby sources reject that: Bulk Reef Supply says "The bulbs do not contain reproductive spores," and Reef Builders that popping them "doesn't release 'spores' which go on to produce millions more." That may be too absolute, since mature cells do form zoospores inside them, but nobody has shown popped bubbles seeding a tank with spores.

The research points to a different risk. In the only controlled study of bubble algae control we found, Figueiredo and colleagues filmed emerald crabs feeding at high speed. As the crabs tore the cells, "the cell liquid that contains juvenile cytoplasmatic spheres is released into the water," and the authors wrote that "this behaviour might contribute to algal dispersal and consequently algal infestation." Lab work backs the idea that a broken cell isn't dead material. In a 2011 study, protoplasts "were artificially induced in three species by cutting thalli and extruding the protoplasm"; they built a new cell wall within 24 hours and went on to develop into mature plants. An earlier study grew cells regenerated from Valonia utricularis protoplasts that stayed viable for several months.

So the advice to avoid bursting bubbles in the display holds, for a different reason than spores. Nobody has measured how often a burst bubble starts new growth in a home tank, and AlgaeBarn's point is fair: an accidental rupture "does not mean your reef is doomed." If one bursts, siphon the spot straight away.

How to get rid of bubble algae by hand

Manual removal is the method every source agrees on. BRS says to act at the first bubble and to repeat removal "on a very regular basis."

1. Turn the wave pumps off. Reef Builders: "the bubbles will only float around and attach to the pump cage."
2. Lift each bubble off at its base. Use tweezers, or a metal tooth scraper for tight crevices (Reef Builders). Work under the bubble rather than squeezing it.
3. Siphon as you go. BRS suggests siphoning during water changes; holding the hose next to the bubble catches anything that comes loose or bursts. Net or siphon any free-floating bubbles.
4. Scrub the spot. A toothbrush removes leftover fragments and rhizoids (Reef Builders).
5. Take bad rocks out. If a rock that can come out is covered, hobbyists clean it in a container of tank water outside the display and then put it back, so fragments don't stay in the tank.
6. Check again every week. New bubbles are easiest to take out while they are small.

A UV sterilizer won't help with this. UV only treats what flows through the chamber, and bubble algae is attached to the rock.

Emerald crabs: what the study found

The emerald crab (Mithraculus sculptus, sometimes sold as Mithrax) is the usual biological control. LiveAquaria says it "will eat bubble algae," lists it as reef compatible but semi-aggressive, up to 2½ inches, and warns that "this opportunistic feeder may turn to corals, invertebrates, or small fish for a food source." It doesn't tolerate copper-based medications.

Figueiredo and colleagues (2008) tested how well it works when other food is around. Medium and large crabs ate more bubble algae than small ones. Given a choice, the crabs mostly took pellets or frozen mysids over algae, and "algal consumption only decreased significantly if, besides algae, pellets were provided." Their conclusion: the crab "might not be such an efficient bio-controller of the pest V. ventricosa as previously thought," though "its bio-control efficiency might be improved if, mysids are used as food."

That matches hobby experience. Reef Builders says "a well-fed Emerald crab is less likely to want to eat Valonia," that more crabs and less other food work better, and warns of "the odd-cheeky emerald grabbing at fish and turning on Zoas." BRS says crabs are often seen eating bubbles "once other more palatable algae are no longer available in the aquarium." In a Reefs.com thread, some owners watched their crabs eat it and others never saw them touch it.

In practice: buy medium or large crabs, take out the existing bubbles by hand first, and use the crabs to pick off new ones. Go easy on pellet food in that tank, but don't starve the crabs, which is when they turn on corals and fish. Their tearing may release cell contents, so a crab is a way to keep bubble algae down, not a cure.

Fish that eat bubble algae

We found no controlled study of fish eating Valonia, only hobby reports, and individual fish vary.

• Foxface rabbitfish (Siganus vulpinus): Reef Builders suggests one "If your tank is five feet long and holds 100 gallons or more," based on reefkeepers' reports. BRS calls it effective but notes it grows to 8 inches. LiveAquaria lists a 125 gallon minimum and venomous dorsal spines, and rates it reef compatible "With Caution": an underfed fish may nip some LPS and soft corals.
• Tangs: in a Reefs.com thread, owners reported yellow, regal (blue hippo) and sailfin tangs eating bubble algae, while another found a Red Sea sailfin barely touched it. They are big fish: see the yellow tang (100 gallon minimum, per LiveAquaria) and blue hippo tang (about 180 gallons and a 6-foot tank) care sheets.
• Vlamingi tang: BRS calls it an incredible algae eater, but it grows to 24 inches and is "best reserved for 200+ gallon aquariums."

If your tank can't house one of these fish for life, it isn't a bubble algae option.

Nutrients and light

BRS says algae grow faster in nutrient-rich water, so keeping nitrate and phosphate low helps slow bubble algae. But eliminating them "will adversely affect your coral," and since bubble algae can thrive even in good water, starving the tank won't remove what is already there. AlgaeBarn makes the same point: "an ultra-low-nutrient aquarium isn't automatically a healthier one."

Keep nitrate around 2–10 ppm and phosphate around 0.02–0.10 ppm. If phosphate is high, bring it down no faster than 0.5 ppm a day, the limit the phosphate page explains; the GFO calculator spreads the drop for you. Driving both to zero trades bubble algae for a worse problem: zero-nutrient tanks are where dinoflagellate outbreaks are most often reported.

Light is not a lever either. The Smithsonian notes that bubble algae tolerates low light, and we found no source showing that a blackout kills it.

Chemical treatments

We found no product labelled specifically for Valonia. The one BRS mentions is API's AlgaeFix, which it says "is known to kill off Bubble Algae with repeat usage." BRS treats it as a last resort, because it also kills other algae, including macroalgae, and "has been known to irritate corals." API's Marine AlgaeFix, the version labelled for saltwater, says it won't harm corals or invertebrates when used as directed.

If you use it, follow the label exactly: 1 mL per 10 gallons, repeated every 3 days until the algae is under control, then one dose a week, with dead algae siphoned or scraped out. API says "Do not overdose," and the product is harmful if swallowed or absorbed through the skin, so wear gloves. The label also states "It is a violation of federal law to use this product in a manner inconsistent with its labeling."

What not to do

• Don't burst bubbles in the display, or pierce them with a syringe. Either releases the cell contents into the water.
• Don't strip nitrate and phosphate to zero to starve it. BRS warns that harms coral, and it invites dinoflagellates.
• Don't count on a blackout or a UV sterilizer to remove attached bubbles.
• Don't starve an emerald crab to make it eat algae; hungry crabs turn on corals and small fish.
• Don't buy a foxface or tang for a tank smaller than its minimum.
• Don't dose hydrogen peroxide or other unlabelled chemicals into the display for bubble algae. Use a product only as its label directs.

For the other common nuisance algae, see the algae identification page; for a tank with nitrate that won't come down, see high nitrate and the water change calculator.