Getting up one fine morning to a cloudy green fish tank is the worst nightmare of any fish owner. Last night, everything was fine. Why is my fish tank suddenly green this morning? How will it affect the fishes?

All these thoughts flee through your mind as you immediately change the tank water, replace the filter cartridge, and are happy to see your fish tank back in its glory.

However, after 2-3 days, the aquarium water again turns greener (than before) overnight. A few hobbyists have even given up their passion due to such experiences.

A sudden algae bloom in your fish tank is the culprit. Have you heard about it? Well, there is always a first time. Besides, prior knowledge empowers you to avoid/manage it effectively in the future.

 In this article, we will explain everything about algae, algae bloom, its causes, and ways to prevent it.

What is Algae?

Algae are aquatic plants. Unlike other plants, they do not have well-defined organs like branches, stems, flowers, etc. They can be single-cellular or multicellular.

Several algae are not visible to the naked eye. They grow in water bodies using sunlight to make their food through photosynthesis. Thus, algae grow in oceans, seas, rivers, streams, lakes, ponds, and aquariums. They can grow in both freshwater and marine water fish tanks.

Is Algae Good for Fish

Alga is a type of plankton that grows in marine and freshwater.

Planktons are floating matter in the water. They are microorganisms. However, some of them (like copepods and some jellyfish) are visible to the naked eye.

They swim on the surface of water bodies to access sunlight for photosynthesis. Some are present in the ocean depths (benthic zone) as marine snow.

Planktons (and algae) in healthy proportion are beneficial as they:

  • Complete the natural ecosystem as producers, consumers, or decomposers in the food web.
  • Consume dissolved carbon dioxide and produce oxygen during photosynthesis. It oxygenates the water.

Planktons can be classified according to their anatomy. But to understand their role in supporting aquatic life, we will go through a few classifications based on their placement in the food web (producers, consumers, etc.)

Below are the types of plankton in the food web.

Likewise, the quality of algae in your fish indicates the health of your aquarium water.

Golden Algae, Brown Algae, Red Algae, and Green Algae are types of phytoplankton.

Seaweed is a type of plankton too.

  • ZooplanktonThey are consumers in the food web. They cannot produce their own food, so they eat phytoplankton and the eggs/larvae of other organisms.
    Copepods and Amphipods belong to this category.
  • Bacterioplankton– They are the decomposers. They eat the waste of other organisms, completing the food chain. They Remineralize the water.
  • CyanobacteriaIt is blue-green algae. Although it is called algae, it is a bacteria. Its anatomy is different from phytoplankton. This bacteria is also present in aquariums.

Did you know? Some creatures live their entire lives as planktons. While a few live as plankton in their initial stages and metamorphose as fishes, worms, or crustaceans.

Do Algae Filter Water

Yes, algae act as a biological filter in water bodies. They complete the carbon cycle, as explained above. Along with other good bacteria in the water, algae help reduce toxic chemicals like nitrogen in aquarium water.

Let us understand how toxic chemicals are formed in your aquarium water.

Aquarium Ecosystem

  • The bioloads (fish poop and urine) accumulate in the aquarium water. These consist of ammonia.
  • When ammonia levels build up, bacteria that consume ammonia colonize aquariums automatically, with time. They are called good bacteria.
  • In turn, they release nitrite in the fish tank water.
  • Now, there are two chemicals in the water – ammonia (produced ongoingly and nitrite (due to the recycling of ammonia).
  • Both these chemicals are toxic and can poison the fish.
  • Once the nitrite levels build up over time, other good bacteria that consume nitrite colonize the aquarium.
  • They release nitrate after consuming nitrite. Nitrate is a less toxic form of nitrogen.
  • Live plants consume nitrates as fertilizers. Hence, adding them helps cut the nitrate levels further.
  • Uneaten food and other detritus also produce phosphates in the water.
  • Algae grows along with good bacteria. It grows automatically due to the presence of nutrients like nitrate and phosphate.
  • As discussed earlier, it reduces toxic chemical levels. Besides, larvae, fish fry(s), and omnivorous fish(es) eat algae.
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When this natural ecosystem stabilizes, the fish tank is ready to house fish(es).

Regular maintenance (refer to section 4.0) is essential to reduce organic waste and keep toxic chemical levels in check.

The images below depict this ecological system of an aquarium.

Image Credit: commons.wikimedia

What is Algae Bloom in a Fish Tank

Image Credit: reddit user : kdbfisherman

Algae bloom occurs when there is a sudden spike in the population of algae in the water bodies.

Algae (phytoplankton) is always present and floating in the fish tank. However, it is invisible to the naked eye. It can multiply rapidly in a sudden algae bloom.

 It makes the aquarium water greenish. Hence, it is also called a green water outbreak.

Harmful Algae/Algal Bloom (HAB) refers to algae overgrowth in water bodies.

Algae bloom can occur in any fish tank (viz. saltwater, tropical, Reef/Coral, and freshwater). 

What Causes Algae Bloom in a Fish Tank

Possible causes of algae bloom are listed below.

  • Excessive light – When aquarium water receives long photo periods (sunlight or artificial light), it induces algae growth. Algae produce their own food using photosynthesis and multiply.
  • High level of nutrients in the aquarium – nutrient refers to nitrates and phosphates (which provide nutrition to algae). If the aquarium ecosystem is disturbed, it increases toxic chemicals, inducing excessive algae growth.
  • Over-cleaning the aquarium – As per section 1.3, it takes time for an aquarium to complete a nitrogen cycle. During this time, the required amount of good bacteria and algae are well developed in the water to cut the toxic chemical levels.

Too thorough a cleaning will remove the good bacteria as well. These will take time to grow back, resulting in high nutrients after cleaning, leading to an algae bloom.

  • Water changes – They play a vital role in maintaining the aquarium’s water quality. However, a 100% water change removes good bacteria, and the presence of heavy minerals in replacement water can result in high algae growth.
  • Poor tank management – improper or lack of filtration, lighting, water changes, excessive uneaten food and decaying matter in the tank bottom, overcrowding, etc., result in high nutrients in the aquarium water, causing algae bloom.
  • Adding a lot of new fish(es) in one go spikes the bioload immediately, which existing good bacteria cannot manage, increasing algae growth in the tank.
  • At times, although the tank is well-oxygenated, a certain part does not receive the required water flow, inducing algae growth.

How to Get Rid of Algae Bloom in Fish Tank

Other things being in place, algae bloom in a new aquarium is likely to fix itself over time as good bacteria increases in the tank water. Keep a few or no fish(es) in the aquarium until the water auto clears.

However, sudden algae bloom in a well-established fish tank needs corrective measures, depending on the cause(s) for it.

Some of the ways to control algae bloom are:

  • Limit exposure to light – Place the aquarium away from direct sunlight and use window shields (like curtains, etc.) to reduce direct sunlight reaching the aquarium.

Around 5 to 8 hours of light is sufficient for any aquarium. Switch off aquarium lights at night. If you have nocturnal tank dwellers, add ample floating plants to minimize light penetration and use dim lighting at night.

Reduce the aquarium lighting time to 3-4 hours a day, or cover the aquarium with a dark blanket or black plastic for 3-4 days to reduce algae bloom in the fish tank. Remove the cover while feeding the fishes. You can skip feeding them twice a week. They will not die.

Blackout aquariums with no plants periodically to prevent algae bloom.

Remember, lack of light exposure can also cause certain types of algae.

  • High nutrients will reflect in physical or behavioral changes (due to stress) in the fish(es). They may look pale, feel breathless, lethargic, bloat, lose appetite, become aggressive, etc.
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Use a good water testing kit to test ammonia, nitrate, carbon, and phosphate levels. Early detection and correction prevent algae bloom.

  • Be careful when cleaning the tank to retain the good bacteria.

Clean the substrate lightly. Change filter cartridges as per the instruction manual. Do not change all the cartridges together. Change 50% to 75% only (for example, one of the two or three of the four) in rotation to retain the good bacteria therein.

Do not perform water and  cartridge changes simultaneously.

  • Change only 10  percent of the water every week or 25 percent every fortnight. Check the replacement tap water for the presence of heavy minerals. If need be, use RO water to replace the water. Dechlorinate the replacement water before performing water changes.
    While treating algae bloom, perform 10% water changes every day, till it is fixed.
  • Efficient tank management – use appropriate lighting, conduct regular water changes, avoid overfeeding the fish(es), and avoid overcrowding the aquarium. Choose a good oxygen and filtration system to clear detritus and oxygenate the tank, add live plants, provide a day/night cycle to the aquarium, etc.
  • Introduce new fish(es) to the main tank gradually, and not in one go. It will give time for the good bacteria build up to control the excess bioload
  • Adding live plants reduces nutrients in the water, preventing algae bloom in the fish tank.
  • Add scavenger fishes/invertebrates (like Kuhli Loaches, Snails, Corydoras, Amano Shrimp, etc.) to keep the tank clean.
  • Algae-eaters (like Mollies, Otocinclus Catfish, Siamese Algae Eater, Chinese Algae Eater, etc.) can reduce algae growth.
  • Using an ultra-violet (UV) sterilizing light in the aquarium prevents algae bloom. It fixes algae bloom in three to five days.
  • Apt changes in the water filter placement and tank decor, increasing the water flow (without impacting the fishes) to cover all parts, or adding aquatic plants in the remote portions (not receiving enough oxygen) can help curb unwanted algae growth.
  • White and blue aquarium lights promote algae growth. Avoid using them to control algae bloom in a fish tank unless you have a thickly planted aquarium to counterfeit it. Red light stops algae growth.
    Set optimum lighting and density using dimmers and adjustable light.
  • Some visible algae stick on the aquarium glass walls or decor. Scrape it immediately. Some algae create a thin film on the surface water or a filmy residue in the tank. Water changes should take care of it.
  • Avoid adding excessive external fertilizers or carbon dioxide injections in the water. Unabsorbed portions will lead to high nutrients into the water.

Kinds of Aquarium Algae Blooms

Many types of algae grow in aquariums. Their popular names indicate their colors (like green algae, red algae, etc.).

There is no alternative to improving tank maintenance to prevent algae bloom in a fish tank. Hence, the same is not repeated in the table below.

This table lists some of the most common aquarium algae, and how to identify and fix them.

Green Water Algae Bloom

It is the most common algae, which turns the tank water green and cloudy. The image on the left shows a green algae bloom and how the tank looks after fixing it.

Excessive exposure to light is the main reason for this algae bloom.

Refer to section 4.0 point (1) for more details.

Add algae-eating fishes like Plecos and Siamese Flying Fox to eat green algae.

Audouinella/Black Beard Algae (BBA) Bloom (type of Red Algae)

The image to the left explains its name. It can be black or gray. When it dies due to corrective measures, it turns red (refer to the right-side image).

Caused by high nutrient and light period, it is one of the most difficult to fix. Amano Shrimp and Siamese Algae Eater eat BBA.

Remove BBA manually. Dip the infected plants and decor in a 1:20 bleach solution periodically. Rinse them thoroughly before placing them back in the main tank. Notch-up tank maintenance. Add the algae eaters mentioned above. Use chemical treatment sparingly in extreme cases.

Brown (Diatom) Algae Bloom

Diatom refers to a type of algae classification based on anatomy. This alga can grow even due to insufficient light. It can even be mustard-colored.

It forms a slimy brown layer on the plants, decor, substrate, or tank walls.

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Remove the algae manually. Clean the infected decor or plants with bleach regularly. Rinse them properly to avoid bleach entering the tank. Increase/decrease the light as need be. Otocinclus catfish and Yellow tangs eat it. Add them, if the issue persists.

Hair Algae Bloom

As the name suggests, it resembles strands of hair. It includes various algae that grow in hairy form (like thread algae, fuzz algae, string algae, etc.).

High nutrients and extended light exposure cause it.

Remove it manually. Take corrective actions to balance nutrients in the water. Add algae-eating fishes like Amano Shrimp, Siamese Algae Eater, Mollies, etc. to curb it.

Blue-Green Algae (BGA) Bloom

As discussed in section 1.0, it is actually a bacteria. It covers the substrate and plant decor with a bluish-green hue and has a typical smell, which can be identified at the initial stages before the bacteria manifests.

Algae-eating fish/invertebrates do not eat this alga. Reduce photo time (this may impact your plants). Scrape it manually. Clean the substrate, perform water changes, and administer an over-the-counter antibiotic that is safe for your fishes. Some may be more sensitive to medication.

Green Dust (GDA) Algae Bloom

It forms a green slime in the aquarium. It is commonly found in new aquariums before the nitrogen cycle (refer to section 1.3) is complete.

Do not confuse GDA and GSA.

In a newly set up fish tank, do not do anything. Once the nitrogen cycle is complete, it will clear out. It is recommended NOT to scrape it, as the dust will initiate a new production of spores, multiplying the GDA. Instead, leave it to die by itself.

However, in a well-established fish tank, this algae bloom will need corrective measures. Refer to sections 4.0 and 5.0 for more info on causes and prevention. Bristlenose Plecos eat GDA. Add them if the issue persists.

Green Spot Algae (GSA) Bloom

True to its name, it appears as small green dots on fish tank walls, slow-growing plants, or decor.

Scrape it manually. Nerite snails eat GSA. Add them if the issue persists.

It can be caused by a chemical imbalance, or improper light exposure. Refer to section 4.0 point (1) for more details.

Frequently Asked Questions (FAQ)

Is algae growth in a fish tank affected by temperature?

Yes. As discussed in section 4.0 above, exposure to light affects algae growth directly.

High water temperatures (above 75 degrees F) reduce oxygen levels, increasing the nutrient levels in the water. Thus, photoperiod and density of light directly impact water temperature, affecting algae growth.

Why does my fish tank keep turning green?

Unless the root cause of the green outbreaks called algae bloom is identified and fixed, it will bounce back with greater intensity. Simply cleaning the tank and changing the water will not treat the algae bloom.

There are many reasons for a sudden algae bloom. Please refer to sections 4.0 and 5.0 above for more details. Once the root cause is fixed,  it will stop.

Remember, proper tank management is the key to preventing algae bloom. It is an ongoing activity.

How long does it take to treat algae bloom in a fish tank?

If the root cause is fixed, it should show results in 3 to 5 days.

A new fish tank can take 6 to 8 weeks to complete its nitrogen cycle. Any cloudiness in the tank during this phase will auto-clear if everything else is in place.

 If you do not see any results, look for other causes besides the one you fixed. Maybe, the corrective measure(s) need proper implementation. Check for similar issues in online fish forums and communities.

Investing in a UV sterilizing light should help.


Algae is beneficial for aquariums, but algae bloom is always bad news.

Stopping algae bloom in a fish tank is easy. Proper tank management and basic hygiene are the golden rules for avoiding algae bloom. Many fish owners have managed fish tanks for years without a single algae bloom, proving the point.

However, if you ever encounter it, do not feel disappointed. You know how to deal with it effectively and prevent it in the future. Share your experience with fellow hobbyists. Sharing knowledge is a great way to connect.

Your fish will always thank you for providing them with a healthy and happy environment.

We wish you a happy fish parenting time with no glooms due to algae blooms!

About the Author

Shelby Crosby

Shelby is a passionate fishkeeper who has been writing about fish for over 5 years. She is a pro aquarist and holds a BSc Honors Degree in Wildlife and Fisheries. She creates her own beautiful aquarium layouts and loves to share her knowledge of tropical fish with other hobbyists.

Career Highlights:

  • Has worked with several aquarium manufacturers as a consultant
  • Organized and hosted workshops on freshwater fish keeping at retail stores, educational facilities, and libraries
  • Released content for the amphibian community through her writings

Educational Highlights:

  • BSc Honors in Wildlife and Fisheries in 2011 (University of Northern British Columbia)
  • Completed her undergraduate thesis on the effects of zoochlorella supplementation on the growth and health of fish.

Writing Experience

Miss Crosby is a Freelance blogger; many of her articles are posted online on various blogs. She has also written a few short articles for "Tropical Fish Hobbyist Magazine" in the past. She is a regular contributor to Her education, first-hand experience with fishkeeping, and in-depth knowledge in aquaculture make her one of the most competent writers in the industry.

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