How to treat algae bloom in fish tank: Green water elimination

green water algae outbreak in fish tank

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How to treat algae bloom in fish tank: Green water elimination

Dealing with the dreaded green water algae bloom in a fish tank is every fish keepers worst nightmares. They can be very hard to diagnosis and treat but after dozens of fish tanks I have come up with a simple solution on how to treat algae bloom in fish tanks.

What causes an algae bloom?

Most “Green Water algae” outbreaks are caused by a sudden population suspended algae known as phytoplankton. Unlike other algae species that grow on the glass or decorations in the aquarium, green water algae float about the tank and multiply by the billions in a short period of time, in what’s known as a “bloom”. They’re typically not harmful to your fish and other aquarium critters, or you, for that matter, but they are unsightly and in severe cases can block light to plants.

Before we can fix an algae bloom we need to determine what caused the algae bloom in the first place. There is a lot of misinformation out there on what causes and algae bloom and a lot of it is wrong. You will here things like natural sunlight causes an algae bloom but I have had fish tank in a basement with no natural light that have had algae blooms and I have had fish tanks right in front of a window that don’t get algae blooms. It really comes down to the water parameters in your tanks and what is out of balance.

In my experience overfeeding fish and uneaten food is the number one cause of algae blooms in my tanks. When I went away from almost all flake foods and only fed brine shrimp or other live foods I did not have any algae blooms. The second leading cause is too much lighting and not enough live plants. Too much light and no live plants to use the nutrients in the water is a good way to get an algae blooms

Green water algae blooms are a common issue for fish keepers. They can be difficult to diagnose and treat, but with experience, there are simple solutions to treat them. To effectively address the problem, it is important to understand the causes of green water algae outbreaks. Unlike other algae species that grow on the glass or decorations in the aquarium, green water algae float around the tank and multiply by the billions in a short period of time, causing a “bloom.” They are typically not harmful to fish or other aquarium critters, but they are unsightly and can block light to plants.

In many cases, overfeeding fish and uneaten food are the primary causes of algae blooms. When fish are overfed, excess nutrients can accumulate in the water and promote the growth of algae. Reducing feeding amounts or switching to live foods, such as brine shrimp, can help prevent algae outbreaks. Similarly, too much lighting and not enough live plants can also lead to an algae bloom. Too much light can promote the growth of algae without the presence of live plants to use the nutrients in the water.

To treat an algae bloom, there are several methods to try. It is important to test the water to ensure that all the parameters are within spec and that there is no obvious issue, such as high nitrate or nitrite levels. Deep cleaning the tank, including all decorations and sides, is also essential. Removing as much uneaten food and fish waste as possible from the water column can help eliminate excess nutrients that can contribute to the growth of algae. Cleaning the substrate thoroughly, including decorations and filter media, can also help reduce the spores of algae in the tank.

After cleaning, doing a massive water change of up to 50% of the tank’s water volume is recommended. The amount of water change may vary based on the aquarium’s size and age. If the tank is less than six months old, it is advised to do only about a 30% water change. After the water change, turn off all lights for at least a week. Algae needs light to grow, and eliminating light can help eliminate the algae. Cutting back on the feeding of fish can also help clear the water naturally. Feed only small amounts, and ensure that all of it is eaten in less than a minute with none falling to the bottom of the tank.

If none of the above methods work, an algaecide may be used. However, this should be a last resort as it can harm the fish. In addition, installing a UV sterilizer can help eliminate the algae outbreak, but it should also be considered as a last resort. Using bottom feeding fish or algae-eating fish can also help keep the tank clean, while live plants can help with algae outbreaks because their leaves and root systems tend to use up many of the nutrients in the water that algae needs to bloom. Adding live plants can make the tank healthier overall and prevent some algae growth.

How to treat algae bloom in fish tank.

There are many different ways to treat an algae bloom in a fish tank and I have tried all of them. From adding algecides into the water to using uv lights I have tried just about everything there is to try out there.

Here is the steps I have found to be the most effective in treating an algae bloom.

Step 1: Test your water

The first step in determining what caused an algae bloom is testing your water. Make sure all the water parmeters are within spec and there is nothing obvious like high nitrates or high nitrites.

Step 2: Clean your tank:

The next step is to do a deep clean of your tank, this means cleaning all the decorations and all the sides of the tank. You want to get as much uneaten food and fish waste out of the water column as possible.

Clean the substrate thoroughly an get as much fish food and waste out as possible. Once that is done you will want to clean the decorations and with some typed of cleaning solution’s like bleach or vinegar. Once you have everything cleaned it is time to put all the decorations back into the aquarium.

You also need to do a deep clean of your filter, at this time you should replace all your filter media like floss and sponges. Also you should rinse off all your biological media in warm tap water to try and get all spores of algae off of them.

Step 3: Do a massive water change

Go ahead and do a big water change up to 50 percent of your aquarium water. I have safely changed up 75 percent of my tank water before with no ill effects on the fish. You can only do this if your tank is established which means up and running for at least 6 months if it is any less than that you should only do about a 30 percent water change.

Step 4: Turn off all lights.

Once you have done a water change and have new water in your aquarium it is time to turn off all the lights. Algae needs light to grow and if you eliminate the light you can eliminate the algae. You should leave the lights off for at least a week. Your fish will be fine and you may see that they are actually out more and swim about because they are not scared of the lights.

Step 5: Cut back on feeding your fish

By cutting back on the feeding of your fish you will find that your water will start to clear up naturally. feed only very small amounts and make sure that it is all eaten in less than a minute with none of it falling to the bottom of the tank.

Step 6: Use an algaecide

If none of the above methods work then you may have to resort to an algaecide. This really needs to be a last resort because I feel like it harms the fish. Every time I have tried to use it I have noticed my fish will not swim around and hang out near filter outlet acting like there is not enough oxygen in the water.

Step 7: Install a UV sterilizer

UV sterilizers will definitely work but they should be a last resort. I am not confident that they also do not remove the good bacteria also. But a good UV light will eliminate a algae outbreak. In my experience the best one are inline UV light that only treat the water that flows through the tubing and not the whole aquarium.

Step 8: Add bottom feeding fish and algae eating fish

Bottom feeding fish like Cory catfish or some algae eating fish like otocinclus will do a nice job of keeping your tank clean. I like the bottom feeder because they tend to eat a lot of the uneaten food and keep your tank water healthier.

Step 9: Add live plants

Live plants can help with algae outbreaks because their leaves and roots systems tend to use up a lot of the nutrients in the water that algae needs to bloom. Adding live plants will make your tank healthier in general and help prevent some of the causes of poor water parameters.

What causes green water algae outbreaks?

Too much light

Algae, being plants, thrive in light. However, when excessive light is combined with an abundant source of nutrients, it can cause an excessive growth of algae, known as a “bloom”. Factors such as placing the aquarium in a sunny location, leaving the tank light on for extended periods of time, or using a light that is too powerful for the aquarium can all contribute to excessive light levels and promote the growth of algae. It’s important to be mindful of the light levels in the aquarium to maintain a balance and prevent excessive growth of algae.


Algae, require certain nutrients to grow, including nitrate and phosphate. These nutrients are often present in fish food, fish waste, and even tap water. An overabundance of these nutrients can occur from overfeeding, having too many fish for the tank size or filter capacity, and even performing water changes with water that contains high levels of nutrients. To prevent excessive growth of algae, it is important to monitor and control the levels of nitrate and phosphate in the aquarium through proper fish feeding practices, fish stocking levels, and water changes.

Bad maintenance

Neglecting regular water changes and proper maintenance can lead to a decline in water quality over time, creating an optimal environment for algae growth. These issues can often go unnoticed, as it is not always visible, until significant problems such as fish illness or an algae bloom occur. To ensure a healthy aquarium, regular water changes and proper maintenance should be a priority in order to maintain water quality and prevent the growth of algae.

The Best way to prevent green water algae outbreaks

Good fish tank maintenance routines

Conducting regular water changes is crucial for maintaining the health of an aquarium and preventing algae outbreaks. A good starting point is to change 10% of the water weekly or 25% bi-weekly. During these water changes, it is important to remove uneaten food and other organic debris by siphoning them out, and lightly vacuuming the gravel. However, it is important to note that this step should be avoided in newly set up aquariums or planted tanks that use substrate fertilizers. Additionally, when adding new water, make sure it is free of nitrate and phosphate.

Another important step in maintaining a healthy aquarium is to regularly service and replace filtration media. This includes replacing hang-on filter cartridges monthly, servicing canister filters regularly, rinsing or replacing mechanical filter media, and changing activated carbon or other chemical media every 4 to 6 weeks. These steps will ensure that the water is being properly filtered, which can help prevent the growth of algae and keep the aquarium’s inhabitants healthy.

The right lighting for your style of fish tank

When selecting a light for your aquarium, it is important to consider the size and water depth of your tank. If you do not have live plants in your aquarium, you do not need a particularly powerful light. To control the amount of light in the tank, consider installing a timer that will automatically turn the light on and off, providing a consistent day/night cycle. Planted aquariums typically require 8 to 12 hours of light daily, while non-planted aquariums can do well with 6 hours or less. It is important to keep in mind that the duration of light can affect the growth of algae.

Overfeeding and overstocking your fish tank

Overstocking your aquarium or overfeeding your fish can lead to an increase in nitrate and phosphate levels, which are ideal conditions for algae growth. To prevent this, it is important to maintain a proper fish-to-water ratio by having 1 inch of adult fish per gallon of water. Additionally, it is important to control the amount of food given to your fish by only feeding them what they can consume in 2 minutes or less, once or twice a day. Many experienced aquarium owners also recommend skipping a feeding day once a week to help maintain proper nutrient levels in the tank.

Live plants

One effective way to prevent Green Water outbreaks, caused by an excess of algae, is to have aquatic plants in the tank. As they use the same nutrients as algae, they can effectively compete and reduce the growth of algae. Additionally, floating plants, such as duckweed and hornwort, can help reduce the amount of light that reaches deeper into the water. Aquariums that are well-planted with a variety of aquatic plants tend to have fewer problems with algae.

Will an algae bloom go away on its own?

Most algae blooms will not go away on their own without changing what cause the algae bloom in the first place. If you turn off all lights and cut back on feeding your fish a algae bloom will go away on its own in time. But if you do not fix what caused the algae bloom in the first place it will quickly come back.

Will a algae bloom hurt my fish?

A algae bloom will generally not hurt your fish. As long as you have water movement in your tank and some type of air being introduced into the water column most fish will not be affected by an algae bloom.

How long will a algae bloom last?

A algae bloom can last anywhere from 1 week to a month with out intervention. With intervention an algae bloom can be eliminated within a couple of days.

Should I do a water change during a algae bloom?

You should do a water change right away when you first notice an algae bloom. Consistent water changes are one of the best way to get rid of algae blooms.

Sources and resources used for this article:

  1. Causes and consequences of harmful algal blooms.” National Ocean Service, NOAA.
  2. “Harmful Algal Blooms (HABs) and Public Health.” Centers for Disease Control and Prevention.
  3. “Impacts of harmful algal blooms on aquatic organisms and ecosystems.” Journal of Environmental Biology.
  4. “Algal blooms: an emerging threat to ocean ecosystems.” Marine Pollution Bulletin.
  5. “Global ecology and oceanography of harmful algal blooms.” Ecological Studies.
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Lee Johnson

Lee Johnson

Aquarium Enthusiast

I love sharing my knowledge about all things aquarium related. I have been keeping aquariums for over 20 years and cannot imagine a life without an aquarium. 

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