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Bryopsis Algae: How to Get Rid of Bryopsis in Your Reef Tank

Unknown fuzzy growths on your rocks are often a source of stress for reef aquarists. Given how sensitive corals can be to algae infestations, it’s well worth getting to know the various types and how to control them.

Bryopsis is one of the worst of the lot and it can easily become so bad that it takes over all of the living space within a tank. Next to no animals will eat it, plucking it only spreads it around, and it even prevents corals from growing nearby. What on earth is to be done about it?!

What is Bryopsis Algae?

Bryopsis is an entire genus of green algae in the family Bryopsidaceae. The two most common species are Bryopsis pennata and B. plumosa but there are several other species that make an unwelcome appearance in reef tanks. While it’s often mistaken for green hair algae, Bryopsis is a pest that’s much more insidious.

The problem with Bryopsis Algae is that very, very few organisms will touch it. It’s noxious due to defensive compounds that make it extremely bitter. A few specialized fish and invertebrates will consume it but rarely in amounts high enough to control or eradicate it.

And once it’s inside your tank, eliminating it entirely is almost out of the question. Hand removal is what most aquarists resort to. But you can never get every single piece from a stalk. And it will not only grow back, but loose fragments scattered about your tank will also settle and regrow. In short, trying to control it can actually make it worse – it’s similar to Aiptasia in that regard.

Bryopsis Algae will grow in and around corals, clams, feather dusters, and other invertebrates we’d prefer to see. It will take up living space that these animals would normally use, preventing healthy reef communities from expanding.

Worse, it also secretes toxins with allelopathic effects towards corals and coral larvae. These chemicals slow and even halt the growth of corals, allowing Bryopsis and other algae to overtake and outcompete them. It’s the worst possible algae to have in your tank and also next to impossible to eradicate…Or is it?

Identifying Bryopsis Algae

Bryopsis has a green fern-like appearance to the individual fronds. The central stalks have branches and from those the green algae filaments spread outwards. It could be pretty if it weren’t such an invasive pest.

Bryopsis Algae is superficially similar to green hair algae but once you’ve seen both they are usually easy to tell apart from one another.

Compared to green hair algae, Bryopsis usually grows much taller. It also has a dense web of holdfasts to attach it to rock whereas green hair algae looks like fluffy growths that leave little to nothing behind. Typical green hair algae also doesn’t have any of the branching structure that Bryopsis does.

Where Does Bryopsis Algae Come From?

Bryopsis Algae doesn’t simply appear in your aquarium like magic, although it can certainly seem that way. It hitches a ride alongside new arrivals to your aquarium. New coral frags and fresh live rock are the most common ways aquarists pick up a Bryopsis infection.

They may not even see the tiny fronds starting to establish themselves on a small piece of solid ground. But by the time you detect it, it’s already shed spores that are establishing in other parts of the tank. Once you have it, you have to act quickly as the thicker it grows in, the more difficult it is to be rid of it forever.

Bryopsis Algae isn’t killed by reducing the lighting for an extended period. It is an algae and is therefore photosynthetic. But so are corals and sea anemones; you’d have to block all light from getting into the tank long enough to kill everything. And even then, the spores left behind would regenerate into new algae once the light returns, only now with nothing else for it to compete against.

Bryopsis Algae can also be triggered to grow spontaneously out of control by nutrient imbalances as well. Phosphates and nitrates are important algae nutrients and are therefore used by corals to fertilize their zooxanthellae; symbiotic algae that live within their cells.

Typically, you’d use frequent water changes or a refugium to act as a nutrient sink and export phosphates and nitrates from your system. A refugium is like something a protein skimmer, only living organisms do the filtration work.

Macroalgae like chaetomorpha and caulerpa absorb nitrates and phosphates before they reach levels high enough for Bryopsis and other problematic algae to take hold. In effect, a refugium boosts the biodiversity of the entire ecosystem, making it much more resilient to sudden shocks.

Controlling Bryopsis Algae

Bryopsis Algae is resistant to trimming, chemical means of control, and very few animals will eat it. Worse, it’s actively toxic to coral growth. How can we be rid of it forever?

Creatures that Eat Bryopsis Algae

Adding algae eaters like Lawnmower Blennies (Salarias fasciatus) or Emerald Crabs (Mithraculus sculptus) to your saltwater clean up crew is helpful with most green algae but with Bryopsis you’re likely out of luck. This algae is noxious not only to corals but animals that would normally try to eat it.

Tangs, Blennies, and Angelfish refuse to try it. Foxfaces (Siganus vulpinus) and other Rabbitfish are your best bet if you plan to rely on fish. Many aquarists report success with these large, mildly venomous herbivores but they may refuse to eat it as well.

Invertebrates known to eat it include Astrea Snails and especially Lettuce Nudibranchs. These are sea slugs that specialize in eating algae of all types. Lettuce Nudibranchs are quite beautiful in their own right and not expensive. A few of them will decimate a Bryopsis Algae infestation but probably won’t eradicate it entirely.

These sea slugs have the ability to take the chloroplasts consumed along with the algae and place them within the frilly extensions of their bodies. They then expose themselves to light, allowing the chloroplasts to photosynthesize and give them a source of sugars to sustain themselves!

Coralline vs Bryopsis Algae

One of the best ways to resist a Bryopsis infection is to encourage a diverse, healthy marine reef ecosystem. Coralline algae are allies in the fight against Bryopsis and other green algae because they use up space that green algae can colonize. They are marine red algae of a different order; Corallinales.

Unlike Bryopsis, Coralline Algae form a hard, calcareous skeleton made of calcium carbonate, just like stony corals. They come in many forms, some of which look very similar to true hard corals. And as a true algae, it competes for the same resources Bryopsis craves: carbon dioxide, light, and algae nutrients.

It also actively encourages the growth of invertebrates that eat algae and shed their outer layers periodically to keep green algae from colonizing them. It’s also been shown that corals actively prefer growing on live rock that has coralline algae already on it.

Once Coralline Algae has become established it’s much more difficult for Bryopsis to get out of control. It does have a few downsides of its own but compared to Bryopsis it’s a massive benefit to most reef tanks.

Chemical Methods to Kill Bryopsis Algae

Bryopsis Algae is resistant to most predators and even algicides tend not to work well on it. That said, there are three proven methods to deal with it.

Using Magnesium to Kill Bryopsis

The most common and effective way to eradicate a Bryopsis Algae infestation is to boost the magnesium levels of your aquarium for 2-3 weeks. Metals like magnesium are actually essential plant nutrients as they help regulate enzyme functions essential for cellular metabolism.

But too much of a good thing will throw the system out of alignment. Past a critical threshold, magnesium starts to interfere with cellular processes. Especially in the production of chlorophyll and the consumption of carbohydrates. Too much magnesium isn’t great for fish or corals either but Bryopsis and other plants reach that critical point much sooner than animals do.

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The majority of reef keepers swear by Kent Marine Tech M Magnesium. There are actually several magnesium additives on the market but reefkeeper wisdom holds that there is an extra component that gives it boosted knockout power against Bryopsis Algae. It’s likely due to the form the magnesium comes in: hydrated magnesium salts.

The trick is to increase the levels of magnesium from their normal reef levels of 1250-1300 ppm to around 1600-2000 ppm. You can safely increase the parts per million concentration by around 100 ppm per day – but feel free to slow it down to 50 ppm if you want to be cautious. Some aquarists report boosting the magnesium concentration by 200 ppm per day with no ill effects to their corals or fish as well.

Make sure that you don’t add any new livestock during this period as the shock of such drastically different magnesium levels may be fatal for them.

Once you’ve reached 1600-2000 ppm, keep the level constant for 2 to 4 weeks and aggressively remove any visible algae. The high magnesium levels will interfere with the algae’s metabolism, killing what remains and preventing it from regrowing.

Make sure you test daily and dose appropriately to maintain your current levels when you perform water changes. Also, be aware that boosting the magnesium levels alters the specific gravity (salinity) of your aquarium water so you’ll need to carefully monitor these changes each time you dose with it.

Bryopsis and Fluconazole

Fluconazole is an antifungal medication used for fish (and human) infections. It’s a general purpose fungicide that’s safe for nearly all organisms…Except Bryopsis and other green algae!

Fluconazole disrupts the chemical pathways these organisms use to create a molecule known as ergosterol. Ergosterol is essential for plants to maintain the integrity of their cell membranes; without it, they literally fall apart on a molecular level.

Dosing Fluconazole is simple and effective; the recommended dose is 20mg per 10 gallons of water. You’ll only need to treat the tank once, making it a much easier method of Bryopsis elimination than continually raising and monitoring magnesium levels. Fluconazole is safe for corals, shrimp, fish, and other animals, but it may kill any macroalgae you have in your tank.

Within a few days, you should see visible damage to the Bryopsis in the tank and within two weeks it will have died back entirely. Since you’re dosing the entire aquarium, you also won’t have leftover spores that will come back to cause problems.

Once you’ve added your dose, don’t perform any water changes unless you redose to compensate for the medication removed. You need to also remove any activated carbon and disable your protein skimmer as they will both remove fluconazole from your system before it has a chance to fully eradicate your Bryopsis.

Hydrogen Peroxide Bryopsis Treatment

The last method you can try for controlling Bryopsis Algae is the simplest and most affordable but not always the most reliable. Hydrogen peroxide is a strongly reactive compound that damages algae on contact. It’s often used as a dip for killing off Bryopsis and other attached green algae to live rock before adding it to your tank.

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The problem is that peroxide only kills off the fluffy green growth. The Bryopsis will likely grow right back over time from the basal filaments that remain attached to the rock. A longer soak has a chance of killing it down to its “roots” but is also harmful to any other organisms living on the rock. You can use standard 3% hydrogen peroxide that’s used as an antiseptic for skin injuries; it’s always handy to have some on hand!

If you detect a few filaments suddenly arising in one spot but nowhere else, you can fill an eyedropper with hydrogen peroxide and “mist” the emerging Bryopsis filaments. You may be able to knock it back hard enough that it dies before it ever becomes established. This way, you avoid having to dose your entire tank with magnesium or Fluconazole.

I wouldn’t use it if there are any corals or other sessile invertebrates directly next to the Bryopsis, however. Also be aware of the currents in your tanks so that it doesn’t get swept directly onto nearby corals.

Hydrogen peroxide isn’t immediately fatal to corals but it is an irritant that will do them harm if it manages to come in contact with them. Fortunately, it dissipates quickly and won’t poison your entire tank.


Bryopsis Algae is the nightmare of marine aquarists and especially reef keepers. It’s tenacious, ugly, and actively poisons nearby corals, preventing them from growing. But with the right animals, ecosystem, and chemicals, you can take back your tank and knock Bryopsis out of the fight forever!

Image credit – Philippe Bourjon, CC BY-SA 4.0, via Wikimedia Commons

Jason Roberts
About Jason Roberts
Jason is an aquarium fanatic that has been a fish hobbyist for almost three decades.

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