Aquarium remedy for blue-green algae (cyanobacteria)

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If you notice a strange blue-green slime growing all over the surfaces of your aquarium, or if you spot a strange smell with no apparent source, you may have an outbreak of blue-green algae on your hands.

So what is this blue-green growth, what is causing it, and how do you finally get rid of that repulsive, slimy mass?

Read this Blue Algae Eradication Guide to find out.

What are blue-green algae?

Blue-green algae are more correctly called cyanobacteria. Cyanobacteria are not algae at all; they are a strain of photosynthetic bacteria that live in water and moist soil. These bacteria are one of the oldest living things on the planet. Some of the fossilized specimens found in Achaean rocks in Western Australia date back over three million years.

The bacteria can be filamentous or unicellular, and some of the species form large colonies that grow quickly to cover the aquarium substrate. If you disturb the algae, it dissolves in leaves that smell unpleasantly fishy or swampy. The algae have a slimy texture. Hence, it is sometimes referred to as smear or slime algae. In severe cases, the algae collect in a foam on the surface of the water. Yuck!

As the name suggests, the algae are usually blue-green, but can also be black, greenish-brown, or even red.

Some types of these bacteria are poisonous to fish and other living things. Fish kills can sometimes also be indirect, as the bacterial colonies suck the oxygen from the water and effectively suffocate the fish. However, there is a type of cyanobacteria called spirulina that is edible and nutritious for fish.

Although cyanobacteria produce oxygen through photosynthesis, they can also convert N2 (inorganic nitrogen) into ammonia. This is very bad news for your fish and puts additional strain on your biological filtration system.

What causes blue-green algae outbreaks?

Cyanobacteria are an incredibly resilient form of life, largely because of their simplicity. The bacteria use light waves that are not used by higher plant species and live on organic waste products such as dissolved nitrates and phosphates, which are often found in abundance in aquariums. Blue-green algae can also occur in a wide range of temperatures.

Cyanobacterial outbreaks typically occur when the aquarium water is high in waste and nutrients, especially phosphates and nitrates. Phosphate in particular contributes significantly to the growth of blue-green algae in an aquarium.

Excess amounts of nutrients and dissolved fish waste in the water are usually caused by infrequent water changes, infrequent vacuuming of the substrate, overcrowding, and overfeeding. Sometimes the problem can arise in new tanks where the biological filtration system is immature and the essential colonies of beneficial bacteria are not yet established.

In some areas, tap water is also very rich in phosphate, which promotes the growth of cyanobacteria in your aquarium. However, the bacteria can still settle in the best-maintained tank.

However, the main causes of blue-green algae outbreaks remain:

  • Introduction to the aquarium on plants or from your water source
  • too much light
  • high levels of organic waste products in the water
  • anaerobic conditions

Unfortunately, cyanobacteria can lie dormant for thousands of years until optimal conditions occur when the blue-green algae bloom.

How to get rid of cyanobacteria

Unfortunately, once inside your aquarium, it can be extremely difficult to remove blue-green algae. However, there are some practical, practical steps you can take.

Take practical action!

  1. You can start by reducing the amount of waste and nutrients in the water, effectively cutting off the bacteria’s food source.
  2. You will also need to physically remove the algae by scraping it off the glass, vacuuming the substrate, and carefully removing the algae from plants and stones.
  3. You should also perform a partial water change of around 25% and turn off the tank lights for three days.
  4. On the fourth day, turn the lights back on and change another ten to 15% of the tank water.

This practice should decrease the amount of algae growth and remove the nutrients that support it. If there is still a problem with the general growth of cyanobacteria or algae, then you need to repeat the process. It's important to understand that if you don't permanently address the underlying causes, the blue-green slimy growth will return.

Maintain a clean, healthy aquarium by regularly changing the water, vacuuming the subsurface and changing the filter media if necessary.

How to get rid of blue-green algae naturally

While it is possible to eradicate blue-green algae using chemicals, which we will discuss later in this guide, you may prefer a more environmentally friendly approach.

First of all, you need to remove as much slimy algae as possible. You can do this with an algae scraper, siphon, or old toothbrush. Unfortunately, there is no point in recruiting a cleaning crew to help you, as most animals don't eat blue-green algae.

Make frequent water changes, clean your filters regularly, reduce overcrowding, and make sure you don't overfeed your fish. It can also be effective at improving flow through the tank by switching to a stronger filter, adding a drive head, or switching things up by moving equipment and decorations so that flow is not buffered. #

What does blue-green algae eat?

Unfortunately blue-green algae are not eaten by algae-eating fish species, not even by Plecostomus. However, the ramshorn snail is the most effective and efficient creature when it comes to eating cyanobacteria. The main disadvantage is that you would need a very large collection of snails to completely get rid of the blue-green algae in the event of a major outbreak.

If you have a sea or reef tank, the Red Reef Hermit Crab is a useful creature as it happily devours cyanobacteria.

Adjust your aquarium lighting

Cyanobacteria produce their own nutrients through photosynthesis and use the energy from the lights in your tank to turn organic waste into a usable form of food. Because blue-green algae use wavelengths of light that are not used by higher-order plants, you can attack the threat by eliminating or reducing those wavelengths, making it difficult for the algae to grow and spread.

If you have older tank lighting, it is most likely that it now emits light with a slightly different wavelength than at the beginning of its life. As light bulbs age, the light they emit becomes less powerful and has a lower wavelength. This fall in light can result in a light bulb rated at 420 to 460 producing light in the nanometer range of 560 to 620 within a year. And this area from 560 to 620 nanometers is the area of ​​light in which blue-green algae multiply.

If you replace the lightbulbs you currently have in your tank with balanced lightbulbs from 6,400K to 14,000K or actinic lightbulbs from 50,000K, the amount of usable light available to the cyanobacteria is immediately reduced. You can also use a UV sterilizer to eradicate free-floating bacteria that will clear the problem within a week or so. However, replacing the lightbulbs in your setup every nine to 12 months is just as effective and prevents blue-green algae from coming back.

Avoid direct sunlight in the tank

Too much light in the aquarium, especially direct sunlight, is a major cause of the growth of cyanobacteria and algae.

If possible, do not place your aquarium in a place where the sun hits the aquarium at any time during the day. We recommend that you do not have your tank in a location where the sun is shining as this can cause large fluctuations in the water temperature that can harm or even kill your fish.

Lower the tank temperature

Lowering the tank temperature below 76 ° C can slow down the growth of bacteria while attacking it from other angles. However, this is only possible if you are running a freshwater tank, as changes in temperature in a marine tank can cause coral and fish stress, which can potentially damage them.

Use antibiotic treatment

If you'd rather chemically attack the algae, you can try antibiotic treatment.

The antibiotic erythromycin can be very effective against blue-green algae. This drug kills the cyanobacteria and gets rid of the nasty, slimy growth in your tank.

However, erythromycin also kills the beneficial bacteria that your biological filtration system needs to function efficiently. Therefore, use the antibiotic carefully and monitor the levels of ammonia, nitrites and nitrates throughout the course of treatment and immediately afterwards.

Although the chemical treatment approach is undoubtedly performed literally within minutes of adding it to the tank, dissolving cyanobacteria can create highly toxic materials to fish and invertebrates in the environment, which has reportedly resulted in massive fish deaths in some tanks, which again created optimal conditions for a renewed outbreak of blue-green algae.

For these reasons, we recommend that you clear your tank of blue-green algae naturally.

How to prevent blue-green algae in your aquarium

So you've learned that once cyanobacteria are established in your tank, they are extremely difficult to move. Hence, all of the steps you can take to prevent an outbreak of this troublesome form of algae are well worth the effort.

Keep your aquarium clean

A great way to prevent algae of any kind from getting in is to keep your aquarium scrupulously clean by thoroughly vacuuming the gravel and partially changing water every week.

If you don't keep the water clean, the nitrates and phosphates will go up, causing many types of bacteria and algae to grow and establish themselves. By simply changing 25% of the water every week, you will keep the nutrient levels low, effectively starve the algae and prevent them from multiplying.

Don't overfeed your fish

Overfeeding your fish is a major cause of excessive nutrient levels in the water, which can contribute to the growth and spread of algae and cyanobacteria.

Most species of fish only need two feedings a day. Feed your fish just enough food to keep your fish busy for a few minutes. If you still see food on the substrate after five minutes, you are overfeeding your fish.


The cyanobacteria lived on nitrates in the tank water. Once removed, you can expect nitrate levels to rise. This is caused by the beneficial bacteria in your biological filter that are replacing the cyanobacteria in the tank ecosystem. Make sure you perform an additional 10 percent water change every other day for up to two weeks to bring the nitrate levels back to the safe range below 20ppm.

Final thoughts

Blue-green algae are a real unsightly nuisance in the aquarium and cover your decorations and your subsoil with a layer of disgusting slime that is difficult to move.

You can get rid of cyanobacteria by keeping your tank super clean and keeping the water in pristine condition as well as limiting the amount of light the tank receives, especially direct sunlight. Make sure you don't overfeed your fish or overfill your tank to limit the number of available nutrients that the algae can feed on.

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