Black algae in an aquarium: understanding, limiting, and preventing

Are you looking for an effective way to limit black algae in your aquarium? How can you limit black algae (cyanobacteria) in an aquarium? Although resistant and aggressive, these particular algae are not impossible to control. Let's explore the different ways to prevent or limit black algae.

At heart, the presence of algae in an environment like an aquarium is an entirely natural phenomenon. Also known as blue algae, they tend to disappear within a few weeks after setting up the aquarium (nitrogen cycle).

It is important to regularly test your aquarium water, and we have asmall guide to discover here on aquarium water testing. Below is a test kit we recommend in most cases to all our aquarium owners.

It already allows you to identify water quality and act accordingly to slow down the growth of algae in an aquarium. You can also add aquarium bacteria that will speed up the development of the nitrogen cycle. 

These bacteria will help reduce the level of organic matter that promotes the growth of algae in an aquarium. We recommend that you perform a small water test. 

When black algae proliferate, it means your setup has a problem (deficiency, excess nutrients, overpopulation, ...). The issue can be easily identified by performing a suitable water test + adding aquarium bacteria. Also remember to provide an aquarium filter suitable for your water volume and properly cleaned. 

Indeed, black algae are more harmful, even more ferocious for plants like fish. They make aquarium maintenance more difficult, and can eventually become completely established and completely invasive. 

You may also be interested in: how to clean algae in an aquarium

What are black algae?

Simply put, black algae are what we call cyanobacteria or Cyanobacteria. They are therefore microorganisms that grow in aquatic environments.

"These cyanobacteria can grow pretty much anywhere, in fresh water or seawater. They are photosynthetic species, creating energy in the form of carbohydrates from sunlight."

They contain chlorophyll, in particular, which gives them their blue/green appearance. These types of algae can also be composed of other water-soluble pigments: these become black when mixed with the blue/green color.

In practice, filamentous black algae do not just invade the decor. They also establish themselves in the leaves of aquatic plants, which they eventually destroy.

Here are some products that work very well to limit algae: 

• AlgExit from Easy-Life (choose according to volume)
• Esha Protalon - The product used by aquarium purists 
• The JBL Range - Essential brand for aquarium care

The growth of these algae indicates that the aquarium ecosystem is out of balance. Unlike green species, which are less harmful, you will need to be quite reactive with black algae. Indeed, you must limit them quickly or risk having your tank destroyed.

Black algae can be found in several types of aquariums, even those designed for aquascaping. Generally, black algae appear in aquariums containing a large number of fish, with or without plants. To complicate things a bit, black algae have several shades of color: sometimes, this makes their identification a bit more laborious.

How to identify black algae in an aquarium?

Inside the aquarium, black algae can take various forms. Thus, you will sometimes find filamentous black algae and, at other times, brush black algae. These invasive plants also take the form of rather slimy black dots on surfaces, walls, or gravel.

If black algae are so resistant, it is because they have an outer coating that protects their cells. They are also called filamentous algae or beard algae, because the tufts are identical to a beard or a brush.

Beard algae or cyanobacteria: make the right diagnosis

In common language, we talk about "black algae" for two different organisms, which explains why this article mentions both beard algae (or brush algae) and cyanobacteria. Both benefit from an imbalance in the water, but they do not present themselves in the same way and do not react to the same levers. Taking two minutes to recognize them avoids unnecessary treatments.

You may also be interested in: our selection of oxygenating plants to keep a balanced pond.

What promotes the growth of black algae in an aquarium?

In an aquarium, the appearance or proliferation of black algae can be caused by different reasons. Below, we have listed different parameters that could promote the growth of these algae, as well as the corresponding solutions.

1. Strong water contamination (bacteria to reduce matter). 
2. An irregular and insufficient CO2system
3. Lack of water softness

1. Strong water contamination

An alarming proliferation of black algae can be explained by a high organic contamination of the water. Generally, this contamination is caused by overfeeding and a lack of regularity in water changes. In other words, the risks of invasion increase if your water is not renewed frequently. This is also the case if the tank is not properly maintained.

An alkaline pH exceeding 7.5 and high carbonate hardness above 13°KH or 25°THCa strongly promote their growth. In this case, the more dirt your aquarium accumulates, the more nitrates form and the greater the risk of a black algae invasion. You can thus simply clean the residue stuck to the substrates using a suitable cleaner (siphon).

2. Lack of regular carbon dioxide

In other cases, the growth of black algae in aquariums is caused by the lack of a CO2system. Yet, it is very beneficial for a good balance. This lack of carbon dioxide allows algae to take all the carbon necessary for their survival and more quickly than slow-growing plants. 

They are also capable of dissociating carbon from the bicarbonate ion, which will considerably increase the pH. It is a whole process that helps algae strengthen their cell walls, which makes them more resistant.

3. Lack of water softness

An aquarium with hard water inevitably attracts black algae. You can thus check the condition of your water using JBL drop tests, KH, and GH. These two tests are of equal importance: they have the same qualities to precisely determine the condition of your water.

How to limit black (blue) algae in your aquarium?

Depending on preferences and context, there are several ways to get rid of black algae. Before that, it is even possible to prevent their appearance by adopting regular maintenance habits. That said, you can opt for algae treatments or choose to limit them naturally.

Here are some products that work very well to limit algae: 

• AlgExit from Easy-Life (choose according to volume)
• Esha Protalon - The product used by aquarium purists 
• The JBL Range - Essential brand for aquarium care

Eliminating the factors promoting algae growth

The proliferation of algae also depends on your feeding habits for the aquarium inhabitants. If you are used to using liquid fertilizer, you must temporarily stop or greatly reduce the dose. It is also important to avoid overfeeding by reducing the amount of food.

Regular aquarium cleaning

It is important to clean your aquarium regularly and thoroughly. This maintenance includes changing the water weekly to prevent dirt from accumulating. 

You should also clean the substrate and filters, brush the decorations, and clean the glass. Of course, you must clean the bottom of the tank using a siphon. Make sure to remove all leaves that have served as prey for the algae.

Natural solutions to limit algae 

Some of our customers have managed to limit cyanobacteria by plunging the aquarium completely into darkness for 4 to 5 days. This helps break the development of photosynthetic algae. There are natural solutions such as adding herbivorous fish or snails that will help limit the progression of bacteria.

We recommend using aquarium bacteria preventively and curatively. The most powerful are the bactocool bacteria developed by the University of Ghent. These bacteria will help limit the development of algae in the aquarium. 

Using products to limit algae

In the event of an invasion of black algae, it is recommended to use treatments based on hydrogen peroxide (H2O2) or glutaraldehyde. However, only use this treatment when natural solutions do not work, so as a last resort. 

Effective solutions to limit algae in aquariums: 

• AlgExit by Easy-Life (choose according to volume)
• Esha Protalon - The product used by aquarium purists 
• The JBL Range - The essential brand for aquarium care

The concentration used must remain low (3% solution) to avoid harming your plants and animals. After this treatment, some plants may temporarily change color. They should normally gradually regain their natural appearance, as this is a completely normal phenomenon.

Using black algae eaters for aquariums

The only freshwater aquarium fish capable of eating black algae is the crossocheilus oblongus. However, it is very large, which does not necessarily make it easy to keep in all aquariums. It can only fit in a 200-liter tank, or even larger!

There are also Amano shrimp, which play a major role in limiting this type of algae. However, make sure that the calcium concentration does not exceed the average, otherwise, even these animals will be unable to do anything about it. 

Step-by-step action plan

To move from theory to practice, here is the order in which we recommend acting. The idea is simple: remove what is already installed, then deprive black algae of what feeds them, and only resort to treatment as a last resort.

1. Identify the type of algae. Fixed clumps or a slimy film? Use the comparison above to guide your actions.
2. Test the water. Nitrates, phosphates, pH, and KH at a minimum. Our aquarium tests allow for precise monitoring.
3. Remove mechanically. Cut back heavily affected leaves, brush the decorations, siphon the bottom and cyanobacteria mats during the water change.
4. Correct the causes. Regular water changes, reduced feeding, filter maintenance without sterilization, monitored phosphates.
5. Strengthen the biology. Add aquarium bacteria and promote healthy plants, which compete with algae for nutrients.
6. Treat as a last resort. If the problem persists despite everything, choose a product suited to the type of algae and follow the instructions to the letter.
7. Monitor over time. Test the water and compare your photos regularly: the regression is gradual.

If the pH and hardness of your water are high, our article GH and KH in aquarium water will help you interpret your measurements. And for other types of algae, see I have algae in my aquarium, what should I do?

Common mistakes when dealing with black algae

When looking for "how to remove black algae", you are tempted to act quickly. Here are the reflexes that most often delay the return to balance.

• Treat before identifying. Beard algae and cyanobacteria do not react to the same levers: the finger test, above, takes two minutes. Our article on cyanobacteria in aquariums details this case.
• Clean everything on the same day. Decorations, filter, and bottom all at once can weaken beneficial bacteria. Space out the cleanings and rinse the filter in aquarium water, not tap water.
• Do not measure. Without testing, you are correcting blindly. Simple monitoring of phosphates, pH, and KH shows what really feeds the algae: see our water analysis tests.
• Feed and fertilize as before. An overly generous ration or an overdosed liquid fertilizer provides what the algae are looking for.
• Combine treatments. Several products at the same time add risk to shrimp, invertebrates, and plants, without accelerating the result.
• Expect an immediate effect. Rebalancing happens step by step: follow your photos and tests from week to week.

What should you take away from all this? How to limit them?

All aquariums contain algae, even if they are not all visible. But when these pests grow to the point of impacting the well-being of your plants and animals, they must be limited. 

One of the most effective solutions is the use of bacteria that will compete with cyano and black and blue algae. These bacteria will compete with the algae. 

One of the safest ways is prevention. The more you adopt the right behaviors from the start, the more you will protect your tank. But black algae are difficult to control; so, you can use chemical treatments to remove them. It is also possible that the algae do not completely disappear after the process, but at least, you will have succeeded in slowing their growth.

Frequently Asked Questions