How to get rid of reef tank cyanobacteria (blue-green algae)

Fish tanks can be a wonderful addition to your home. They serve as both a decoration and a hobby to take your mind off things. But as you walk past your aquarium one day, you notice it’s covered in unsightly sludge. What’s more, the whole tank gives off a stench so foul you barely hang on to your consciousness. Will your aquarium recover from this menace, or is it time to bid it goodbye?

In this article, we’ll show you what organisms eat reef tank cyanobacteria (aka blue-green algae) and other ways you can get rid of them.


The best way to get rid of blue-green algae, or cyanobacteria, in a reef tank is by using antibiotics.

How to get rid of reef tank cyanobacteria (blue-green algae)

Unfortunately, barely any animals will touch Cyanobacteria. It means you can’t just insert animals to eat the blue-green algae. The very few that do are all marine slugs. We recommend those from the Trochus and Cerith families if you own a saltwater tank. But if you keep a freshwater aquarium, you’re out of luck. While fish eat algae, Cyanobacteria are different organisms altogether.

But that doesn’t mean you’re helpless against them. You can treat Cyanobacteria by using antibiotics to get rid of it. They are particularly sensitive to erythromycin.

Other than that, you can employ several prevention tactics. Firstly, you should limit the amount of light that enters your aquarium. Too much light promotes photosynthesis and may lead to the proliferation of these bacteria. You should also monitor the tank levels of nutrients such as:

  • carbon dioxide
  • nitrogen
  • and phosphorus.

These nutrients generally accumulate when you don’t clean your aquarium regularly.

How to Remove Red Algae from Saltwater Tanks

Red slime algae in saltwater tanks are actually a type of Cyanobacteria. They often arise because of poor maintenance. Common causes of this issue are:

  • excessive lighting
  • too much organic waste
  • and poor water flow.
150 Gallon Aquarium

The best way to remove them is by using erythromycin, an antibiotic that kills Cyanobacteria. But while such products will kill them, they won’t prevent their regrowth. The best way to deal with these bacteria is prevention. Make sure you change the water and clean your aquarium regularly. Doing so removes most of the phosphate-rich organic waste from the tank. You should also reduce the amount of light that enters your tank. And finally, make sure the water in your tank flows steadily since these organisms thrive in stagnant waters.

You can also introduce red slime algae eaters to your tank. Most types of hermit crabs are a good option. Other saltwater algae eaters include sea snails from the Trochus and Cerith families.

Type of algae in an aquarium

Blue-Green Algae (Cyanobacteria)

While similar conditions cause most types of algae, some specifics vary from one species to another. The natural predators of these aquatic organisms are also specific to each species. In this section, we will introduce you to Cyanobacteria and other types of algae. Apart from physical descriptions, we also provide some prevention tips for each species.

Blue-Green Algae (Cyanobacteria)

Although we commonly call them blue-green algae, these are a type of bacteria capable of photosynthesis. It survives even in low light conditions. The key factor to monitoring its growth is maintaining stable levels of nitrogen and phosphates in your tank.

Cyanobacteria normally appear in most tanks during the initial setup period. It is because the ecosystem hasn’t stabilized yet. If it appears years later after you set up your tank, it often points to a poorly maintained aquarium.

Products that can deal with Cyanobacteria include erythromycin. However, this doesn’t resolve the underlying issue. To prevent it, you need to keep nitrogen levels low. You can achieve this by cleaning the gravel bed regularly. Organic matters release nutrients as it decays, prompting the growth of Cyanobacteria.

Black Brush Algae

Though blackish in appearance, this species is a type of filamentous red algae. It covers the surface of aquatic plants, tank hardware, and underwater decorations. You can identify it by the dark carpet-like patches it forms. Unfortunately, this is one of the toughest specimens to remove. It stubbornly clings to the infected surfaces even when dead.

Once it appears, your best bet is to trim affected leaves. Prevention includes maintaining proper carbon dioxide levels. Liquid carbon additives can be a big help in this department. Some fish, like the Siamese algae eaters, can help you prevent unstable growth of this specimen too.

Blanket Weed (Cladophora)

Blanket Weed

This type of green algae often grows attached to surfaces. However, it can also form floating mats when it grows out of control. Blanket weed relies on carbon to thrive. Maintaining its levels can help you manage its growth. Additionally, you can manually remove existing mats. When it comes to its natural predators, Amano and Cherry’s shrimps are your best options.


These planktonic brown algae also cover the surfaces of your aquarium. Apart from the standard nutrients, they also need silicates to thrive. It is because they form a silica ‘shell’ around their cells. While they commonly appear in newly set-up aquariums, they should disappear as the ecosystem stabilizes. Diatoms shouldn’t appear in well-maintained fish tanks.

The best type of fish to combat diatoms is Otocinclus (dwarf sucker).

Green Dust Algae

This species often cover glass and bogwood. It tends to grow in tanks that get too much light. An imbalance in nutrients might also contribute to its growth.

Prevention includes managing the amount of light that enters your tank and keeping the nutrients balanced. Fish that can help you control its growth include dwarf suckers and bristlenose plecos.

Green Spot Algae

These planktonic species grow on the surface of aquatic plants and aquarium glass. Poor balance of carbon dioxide and other nutrients contributes heavily to its growth. As in the previous case, keeping this in check can help you regulate its growth. Adding Bristlenose plecos and dwarf suckers may also help.

Green Water Algae Blooms

These planktonic algae proliferate extremely quickly. Once it reaches critical levels, the water will turn green. At this point, it will die off. As the dead cells decay, they lower the oxygen levels in the water. It is extremely dangerous as it might kill your fish.

Maintaining proper lighting conditions is the key to its prevention. It’s also sensitive to UV light, though this may harm your fish too.

Staghorn Algae

Also known as Compsopogon, these whiteish algae grow mostly on plant leaves and other surfaces. It branches multiple times as it grows, like antlers. It is how it got its name. You can remove it manually by cleaning the infected surface. Its natural predators are Siamese algae eaters. Of course, you should also follow the standard prevention methods – always keep nutrient levels in your tank stable.

Fuzz Algae

These filamentous species form fuzzy covers on top of your aquatic plants. They often grow in tanks with unstable nutrient balances. Apart from manual removal, you can also introduce fish that prey on these algae. We recommend:

  • Mollies
  • Rosy Barbs
  • Cherry shrimp
  • and Amano shrimp.

Hair Algae

Hair algae thrive in tanks with too much nitrogen and phosphorus. They can be a real nuisance since they tend to get stuck in filter intakes. Manually removing it is not enough, as it can regrow. Make sure you maintain your tank properly. Fish such as Mollies, Flagfish, Cherry shrimp, and Amano shrimp can help you regulate their growth.

Identifying Cyanobacteria

You can identify Cyanobacteria by their unique blue-green color (that’s where they got their name). At first, they form a small distinct spot on the bed of your aquarium. As Cyanobacteria grow, they can quickly cover all surfaces of your aquarium in sticky blue-green slime. However, they aren’t necessarily just blue and green. Other possible colors include black, brown, and possibly even red.

Cyanobacteria are mostly a concern for your plants. They often form colonies on your plants’ leaves, competing for sunlight with them. Sadly, they usually win this competition, and your aquatic plants may die off one by one.

Apart from their vivid color, you can also identify them by their foul smell. You can notice it even several weeks before Cyanobacteria become visible in your aquarium.

What causes cyanobacteria?

Stagnant water

Low oxygen levels often result from poor flow of water. Both algae and Cyanobacteria thrive in such conditions. Consider adding a powerhead or airstone to your tank if you suspect this is the case.

Too much light

All photosynthetic organisms need light to function. But when you always keep the light on, these organisms will start multiplying rapidly. Ideally, your aquarium lighting shouldn’t be on for more than 10 hours a day.

Lack of plant life

Plants need the same conditions as algae and Cyanobacteria to thrive. Introducing them to your tank can thus create a natural competition between these two organisms. Plants will often consume most of the nutrients in the tank, leaving none for Cyanobacteria. Simply adding these plants can help you turn your tank around.

Too much organic waste

Dead leaves, fish feces, and uneaten food accumulate in the water over time. All are rich in nutrients and provide the perfect feast for Cyanobacteria. It is one of the primary reasons why fish keepers struggle with this issue. Don’t underestimate the value of regular maintenance. Changing the water every now and then will reduce the amount of nitrogen and phosphates in your tank. Without these nutrients, Cyanobacteria will die off.

Avoid overfeeding your fish too. Your fish won’t eat more than they need. The uneaten food will then decay and contribute to the growth of these bacteria.


Do plecos eat algae?

Most types of pleco will indeed eat algae. They are one of the best fish you can purchase for this purpose. However, they won’t eat Cyanobacteria.

Do blue-green algae kill fish?

Cyanobacteria are rarely harmful to fish. However, they can kill your aquatic plants. When they grow too much, cyanobacteria often cover the leaves of your aquatic flora. It prevents photosynthesis, causing your plants to die off slowly.

Do hermit crabs eat algae?

Hermit crabs eat many types of algae. Thus, they are an excellent addition to most saltwater tanks. Unlike plecos, they don’t mind snacking on Cyanobacteria.


Though they are called blue-green algae, these organisms belong to a group of photosynthetic bacteria. While they don’t harm your fish, they can wreak havoc on the plant life in your aquarium. It is because they often cover the plants’ leaves and halt their photosynthesis. As a result, your plants will start dying.

Several factors contribute to the overgrowth of Cyanobacteria. The most notable ones are too much light, too much organic matter, and poor water circulation. The best way to treat Cyanobacteria is prevention. Regular maintenance is often enough to limit their growth. You can also introduce some organisms that eat them, but only if you own a saltwater aquarium. Marine snails and hermit crabs are both excellent options.

Once they grow out of control, you can kill them with products containing erythromycin.