What causes algae in fish tanks

What causes algae in fish tanks and how to take it under control

Every aquarium owner will have their own story about dealing with algae. And if you have a tank, you’ll most probably know by now that sometimes it can be difficult to have stable conditions in your aquarium.

So, to make things easier for yourself and the inhabitants of your fish tank, let’s discover what causes algae in fish tanks and what are the most effective ways to get the upper hand in this sometimes, frustrating battle.

10 most common types of algae in fish tanks

Before we dive into what causes algae in fish tanks, it would make sense to start with getting a general understanding of what is an alga. However, finding a single definition is impossible mainly because algae is an informal term used to describe an extensive range of organisms.

A documented fact is that the majority of algae forms are aquatic and present in both freshwater and saltwater environments.

Following this logic, we can quickly figure out that algae are an essential part of any water ecosystem.

But does it mean that every kind of algae is equally suitable for your underwater friends?

To answer this, we are going to look at the 10 most common types of algae found in fish tanks, establish what causes them and explore the ways of getting rid of the unwanted invader.

1. Brown Diatom Algae

Brown diatom will appear as a dusty layer on all the aquarium surfaces in both freshwater and saltwater tanks. Despite the name, this type of algae can be greenish and even fully green.

Although brown diatom algae are not the worst that can happen to your fish tank, letting it develop for too long is not a good idea as it will turn into a slimy coating over every inch of the underwater habitat.

New aquariums are more at risk of getting brown algae due to the filter and substrate not having enough time to establish an adequate chemical balance.

In the older tanks, brown algae can be caused by insufficient lighting and by the streams of direct sunlight.


Luckily, knowing what causes algae in fish tanks in the case of the brown diatom is already a step towards getting rid of it.

Start by physically removing the dusty sheet from all surfaces using an aquarium siphon kit or an algae cloth for the aquarium.

Then, to prevent future development, closely observe the lighting and filtration cycles to make sure brown diatom is deprived of the required nutrients.

2. Green Spot Algae (GSA)

Green spot algae are very easy to spot: it starts as an array of noticeable green dots and can grow into rather extensive coverage areas over time.

You’ll most likely find GSA on the tank walls, decorations, and on slow-growing plants.

For the most part, green spot algae is a result of a nutrient imbalance, specifically the deficiency of phosphates. However, the low CO2 levels, poor water circulation, and too much light can all be reasons for GSA development.


It’s recommended that you equip your tank-tending arsenal with a testing kit.

It will give you a solid understanding of the nutrient balance in the aquarium and help you to make adjustments whenever necessary.

In the case with GSA caused by the lack of phosphate, adding phosphorus to the water will do the trick. Then you can remove the green spots with a scraper.

3. Black Beard Algae (BBA)

Black beard algae are a type of red algae that can appear as bushy, dark patches of hair, primarily in saltwater settings. They are a pain to get rid of both due to their grabby nature and their choice of growing locations.

BBA often appears in tanks with either low or unstable CO2 levels, poor water flow, and inadequate lighting arrangement. Sometimes black beard algae start growing even in the most unlikely conditions even when the water parameters seem okay.

It’s fair to mention that getting rid of BBA can get very tricky and deserves its own discussion.


Among the common solutions, you can go for manual removal with a variety of instruments, treat infested plants with a bleach solution, readjust CO2 levels, rearrange the tank’s interior, or introduce algae eating fish.

4. Hair Algae

Hair algae, also known as thread algae, appear as thin and soft long stems of grass.

Aquarium enthusiasts also confirm that it very much resembles wet hair when removed from the tank, hence the name.

The improper balance between the light and the nutrients, in various combinations, seems to be the primary reason for the development of hair algae. A prevalent cause is a surplus of iron, for instance.

As with any hard-to-remove algae, you will most likely need to employ an array of methods to get rid of the hair algae.

Start by making sure that the nutrient balance is adequate, paying particular attention to CO2.

Then find algae eaters compatible with your fish. The typical hair algae eaters are Amano Shrimp, Dwarf Shrimp, and Siamese Algae Eaters.

A handy little trick for manually removing hair alga is by twisting it around a solid object, such as a pencil or a toothbrush, and then pulling it out.

5. Blue-Green Algae (BGA)

Blue-green algae isn’t an alga, but it is still essential to include it on this list as it frequently terrorizes aquarium keepers and behaves in an algae-like way.

BGA can rapidly cover all surfaces of your tank in a slimy mat, usually greenish-blue, but in some cases purple and even brown.

Blue-green algae thrive in increased organic waste environments that result from consecutive overfeeding and lack of water changes. An abundance of light, as well as insufficient lighting for the plants, can also cause BGA to grow.


Since fighting the blue-green algae invasion is often challenging, there are many approaches to the issue.

Perhaps, the most popular trick is depriving the algae of light by covering the aquarium for several days. If using this method, make sure to place an airstone in the tank, so your fish can get proper oxygen supply during this brief quarantine.

Another solution is to add some fast-growing plants to cut off the nutrient supply to the BGA.

6. Green Water Algae

This type of algae will turn your freshwater tank into something resembling a green smoothie and might look rather dreadful.

Keep calm, the green water caused by single-cell phytoplankton is not toxic for your fish, despite what it looks like.

You know by now that nutrient and light imbalance are often to blame for an algae bloom in fish tanks.

The same goes for the green water type.

But in this case, the algae development is mostly due to unusual spikes in lighting, such as exposure to sunlight, or nutrients—by increasing the fertilizer dose, for instance.


The bad news is that water changes are useless when it comes to a green water situation. Even if you replace 95% of the water, the remaining 5% will be enough to turn everything back in just a couple of days.

What you can try is either a black-out method, which can be harmful to the live plants in the long run or UV sterilization, completed with a special tool.

Additionally, snails can help out in the battle against green water as well as prevent it in the future.

7. Blanket Weed

Ask any aquarium keeper—the blanket weed algae is the hardest to say ‘bye’ to.

Not only does it form a thick woolish looking carpet across plants, substrate, and decoration, but it also releases a nasty smell upon removal.

Sometimes blanket weed appears in balanced, healthy environments, which makes it an extra bit more unpleasant.

However, the broadly recognized source of blanket weed in many cases are low-quality plants, Marimo balls, for example.


As already mentioned, it can be very tricky to get rid of the blanket weed.

If the infestation is small, you might have a chance of plucking it out with stainless-steel aquarium tweezers.

You can also find the anti-blanket weed solutions online but ensure that you are purchasing the aquarium-suitable kind, not the one meant for the ponds.

8. Fuzz Algae

Fuzz algae are often recognized as an early development stage of the hair algae infestation, although it might not necessarily be the case.

Also, in minimal quantities, this short, soft green fluff is beneficial to the tank’s ecosystem and shouldn’t be feared like other algae types.

The new tanks with a not yet established nutrient balance are more likely to have the fuzz algae outbreak.

And in the older tanks, this alga will grow along with low CO2 levels, due to the plants’ incapability to keep the attack under control.


Luckily, many live plants and animals can help to monitor the fuzz. For example, the Amano Shrimp, the Mollies, and the Siamese Algae Eaters.

And your job in this battle is to maintain the nutrient and CO2 levels balanced by frequently testing and adjusting when needed.

9. Green Dust Algae (GDA)

The green dust algae are often confused with the green spot kind. However, while GSA forms small distinguishable circles, the GDA cover the aquarium walls with patches of slime.

Green dust algae occur in new tanks, where the beneficial bacteria forming cycle has not been completed yet and in the environments with low CO2 and nutrient indications.


The methods of fighting GDA are rather passive. In the majority of occurrences, the best thing you can do is to leave the green dust algae alone until it naturally passes through.

GDA lifecycle is done in four weeks, after which you can do a significant water change and wipe off the green spot from the tank walls.

Try not to disturb the green dust while it is still alive to avoid spreading its spores.

10. Staghorn Algae

Concluding this list of different algae and examples of what causes algae in fish tanks is another hard-to-remove subtype of red algae.

From the name, you might have figured that this alga resembles the horns of a stag. It ranges in color from green to blue and dark blue and attaches to pretty much anything in the tank.

Increased ammonia levels in new tanks, low CO2, and water circulation, as well as the notorious overfeeding/skipping water changes combination, all have equal chances of bringing the staghorn algae into your tank.


And in terms of getting rid of staghorn algae, preventing it is a lot easier than removing it.

The popular elimination methods include enhancing the water flow and CO2 levels to boost the live plants’ algae fighting abilities, getting more thorough with the cleaning routine, and, as an extreme measure, treating infested plants with a bleach solution.

Frequently Asked Questions

Now you know a great deal about various types of algae as well as what causes algae in fish tanks. Just before we wrap up, let’s address some of the most frequently asked questions regarding the spread of algae in aquariums.

Can algae kill fish?

Algae don’t kill fish but the conditions that caused the outbreak can kill your fish. Algae become a threat to fish life when it is found in extensive amounts. Some of the possible algae effects on fish are toxic compounds found in some species and, more commonly, the oxygen depletion.

Are green algae good for fish tanks?

Moderate amounts of green algae are beneficial for the tank dwellers in terms of bacteria, minimizing the toxic nitrogens, and completing a healthy diet.

Which fish eats the most algae?

Fish known for great algae eating abilities are the Bristlenose Pecos, the Siamese Algae Eater, the Chinese Algae Eater, the Twig Catfish, and the Otocinclus Catfish.

Does overfeeding cause algae?

Overfeeding, and specifically the increase of dissolved organic materials in the water, is one of the top reasons for algae infestation in fish tanks.

How to prevent algae bloom?

The ultimate solution to preventing algae from blooming is by learning and understanding as much as possible about what causes algae in fish tanks. Water changes, nutrient and CO2 adjustments, lighting, oxygen and water flow rearrangements can all be very effective in avoiding algae bloom.