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green water in fish tank

How to Get Rid of Green Water in Aquariums

If your aquarium water has suddenly turned the color of pea soup, it means green water has infested your planted tank. This happens due to an unexpected population explosion of tiny floating algae called phytoplankton. Getting rid of green water with large water changes often doesn't work because it replicates rapidly. Because of this, we need to use different techniques to take control of the situation and stop its spread.

green water aquarium

In this comprehensive guide, we dive into the issue of green water in the planted aquarium. In addition, there are numerous effective solutions that exist to eliminate green water in your freshwater aquarium.

Let us explain the causes of green water in the fish tank and how it affects the balance of your aquarium's ecosystem. Keep reading to learn how to identify and clear green aquarium water for a healthy, thriving planted tank.

This article will go over:

What Is Green Water?

Have you ever seen an aquarium or pond with water so green that it was nearly impossible to see through? This is a sign of a green water infestation, also referred to as "pea soup algae."

The rapid multiplication of single-celled planktonic algae causes green water. This type of phytoplankton differs from other types of algae we encounter in the aquarium due to the fact that it's suspended in the water column. Its free-floating nature means it can replicate at an extremely fast pace, making the aquarium water look green. Basically, this free-floating algae takes over water, giving it a green appearance.

Green water is a relatively common issue in aquariums. However, it can be frustrating for aquarists as it obscures the view of the aquarium and its freshwater fish. 

green water aquarium

Is Green Water Harmful?

While unsightly, green water is not directly dangerous or harmful to the aquarium ecosystem or its inhabitants. In fact, in controlled amounts, it can be beneficial to your fish and shrimp. Green water naturally occurs in wild, established aquatic ecosystems. Green water contributes to a certain level of murkiness and tint to the water that helps prey species hide from predators.

This free-floating algae can also serve as food for tiny fry and invertebrates. Many breeders purposely create green water cultures because it's a nutrient-rich food source for newly hatched fry. 

In addition, green water also helps combat high nitrate levels in the aquarium. Since algae, like live aquatic plants, is a photosynthetic organism, it feeds off nitrates and reduces toxicity in the aquarium. Similarly, both aquarium plants and green water act as a form of biological filtration, naturally filtering and oxygenating the water. 

Even though green water doesn't directly harm your aquarium ecosystem and its inhabitants, there are still indirect downsides to it.

Most people widely agree that aquarium green water, although natural, is unsightly and lacks aesthetic appeal. Depending on the severity of its spread, it can obscure your view of the aquarium. This can make monitoring the health of your aquarium's ecosystem and inhabitants difficult.

Green water can prevent plants at the bottom of the aquarium from getting enough light. This can lead to nutrient deficiencies and melting leaves.

Like live plants, algae also consumes oxygen and produces carbon dioxide at night in the absence of light. This can cause a depletion of the amount of oxygen in the fish tank, especially in combination with low aeration and a severe case of green water. Low oxygen levels in the aquarium can stress out your fish and shrimp, leading to disease and even death.  

green water

What Causes Green Water? 

Your freshwater tank can experience a green water outbreak if it is exposed to excessive light, an abundance of nutrients in the water column (such as when dosing liquid fertilizer), or a combination of both issues.

It can also happen when there is too much ammonia in a new tank or when you feed the fish too much. Algae is a photosynthetic organism, so it needs both light and nutrients to grow. It can easily overrun an aquarium once these resources become abundantly available.

Scenarios that can cause an aquarium to experience a green water bloom include:

  • Keeping the tank outside or next to a window: Excessive light exposure is an easy way to induce an algae bloom. This is why green water in outdoor ponds is a common occurrence.
  • Leaving the aquarium light on for too long: Too much light from an artificial or natural source gives algae the ability to multiply quickly.
  • Overfeeding the tank: The more food put into the aquarium, the more waste is produced. This waste breaks down into toxic compounds like ammonia and nitrites, which are a food source for the algae to feed off, encouraging its reproduction.
  • Overstocking the tank: An excessive amount of fish in an aquarium leads to an excessive amount of waste produced, causing an abundance of nutrients to be available for the algae to encourage its reproduction.
  • Not cleaning the aquarium enough: Aquarium maintenance decreases the amount of waste buildup and nitrates in the water, resulting in a reduction of excess nutrients for algae to aid replication.  

How to Get Rid of Green Water

Green water spreads fast, so doing water changes alone won't stop it from reproducing. It's important to utilize alternative methods to manage its rate of reproduction.

To truly eliminate green water, you must address the underlying issues causing it. Generally, this approach tackles problems relating to excessive light exposure, as well as excessive nutrients in the water column.  

Here are 6 strategies that can help get rid of green aquarium water:

1. BLACK OUT THE AQUARIUM: Performing a blackout on the tank for at least seven days can stop the proliferation of green water. This easy and non-invasive strategy involves covering the tank up completely from any light. Without light exposure, the algae will die off.
Follow these steps to properly black out an aquarium:
  • First, turn off all lights while keeping all other equipment running. It's important to keep the tank well-aerated during a black-out
  • Second, cover the entire aquarium with a dark-colored blanket to ensure no light can enter the tank
  • Third, perform the black-out for a maximum of 7 days. The purpose is to starve the algae of light so it can't survive. Algae relies on consistent photo periods for energy through photosynthesis. On the other hand, our plants can survive through this period because they have nutrients stored in their roots and leaves to use during this time.
Keep in mind, this method may negatively affect any live plants in the tank as well. A large algae die-off in the aquarium can create an ammonia spike in the water that can be detrimental to the health of your fish and invertebrates. If this is a concern, consider moving any livestock to a temporary holding tank during the blackout. Also, be sure to perform a large water change on the affected tank after the blackout.

2. USE A UV STERILIZER: One of the easiest ways to eliminate green water from an aquarium is to use a UV sterilizer. The sterilizer pulls water through the device, where a UV light then kills off the algae, as well as any other harmful bacterial or parasitic microorganisms. Using a UV sterilizer should clear up green aquarium water in a matter of days. Again, be mindful of ammonia spikes that may result from sudden algae die-offs.
3. USE A POLISHING PAD: Inside a filter, the water polishing pad is positioned after water flows through the main mechanical filtration components. You can add this type of micron filtration to the aquarium's filtration system to remove tiny bits of suspended algae from the water column. The pad's fineness allows it to trap even the smallest bits of debris that stock mechanical filtration components often miss. Examples of polishing pads would be filter floss or filter socks. 

4. REDUCE LIGHTING: If too much light may be the issue, try reducing the total hours/duration your light is powered on. It's recommended to keep an aquarium light on for no longer than 6-8 hours. In addition, consider purchasing a light dimmer for your aquarium light if your light is on the stronger side.

5. FIX WATER IMBALANCES: Conducting regular water tests can unveil imbalances in nitrate and phosphate levels that are triggering algae growth. Ensure you are not overfeeding your tank. Also, consider if there is an overpopulation of fish within your tank that is causing imbalances. To rectify this, either reduce the fish count or provide a more spacious environment in a larger aquarium.

6. KEEP UP WITH MAINTENANCE: Consistent tank upkeep and proper cleaning are vital to thwart the growth of green water and algae. Aim to replace a minimum of 20% of your aquarium water every week, and utilize a calendar alert to ensure you stay on top of these crucial water changes. Additionally, schedule periodic thorough cleanings – you can effectively complete a comprehensive aquarium cleaning within an hour, so make sure to allocate time for these cleanings in your routine.

When these strategies are combined, you'll maximize your chances of effectively clearing your tank of green water. Keep in mind that it may take a couple of weeks of extra care and regular maintenance to see a difference in your aquarium.

How to Prevent Green Water

After successfully removing green water, remember these best practices to keep it from happening again in the future.

Make sure to address any problems related to excessive light exposure, as well as excessive nutrients in the water column. Managing both of these variables will lead to fewer green water outbreaks.  

Here are 4 ways to prevent green water in your tank:

1. ADD MORE PLANTS: Since live aquarium plants also use photosynthesis to grow, they can compete against algae for light and nutrient resources in the aquarium. Green water should slowly die back as the addition of more plants continues to utilize a majority of the available nutrients. More plants lead to an increase in supplemental oxygen in the aquarium.

Fast-growing plants like stem plants and floating plants can prevent green water blooms. Floating plants additionally help by blocking light intensity, inhibiting the algae growth rate even more.

Recommended fast-growing freshwater plants used to consume excess nutrients include Salvinia, Water Sprite, and Hornwort. These three indoor plants float at the water's surface without requiring rooting into the substrate. As long as they have sufficient lighting and oxygen saturation, these plants will consume excess nutrients, ammonia, and nitrates. 
  • Buy healthy plants online here to outcompete algae and restore biological balance to your aquarium.
2. USE PROPER FILTRATION: Ensure that your aquarium filter is large and efficient enough for your setup. In addition, properly cleaning your aquarium's filters at least every 1-2 months will optimize the filtration system's efficiency. 

3. PERFORM REGULAR MAINTENANCE: One of the best ways to prevent green water from returning is to keep up with regular maintenance. We recommend weekly water changes because optimal water quality suppresses algae growth. During water changes, remove as much waste, uneaten fish food, and decaying organics as possible to guard against high ammonia levels.

4. ADJUST LIGHTING: To ensure your aquarium doesn't receive too much lighting, adjust your daytime lighting cycle. Ensure the total hours/duration in which your aquarium light is powered on is set somewhere between 6-8 hours. Also, purchasing a light dimmer will allow you to easily dial in the perfect lighting strength for your tank.

green water

Final Thoughts on Green Water

While green water won't directly harm your fish, it can take away from the aesthetic appeal of your aquarium. This article went into what causes it, its impact on an aquarium, and practical solutions on how to remove it. Clearing green aquarium water is a matter of controlling the amount of light and nutrients your tank gets. From UV sterilization to careful choice of plants, it's possible to keep a beautiful and balanced fish tank.

Remember, the time and care you invest in your aquarium will undoubtedly pay off in the form of a vibrant, flourishing aquatic ecosystem. Keep in mind that every obstacle presents an opportunity for growth, learning, and ultimately, a more beautiful and balanced aquatic environment for both your fish and your own enjoyment.



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Mike - January 17, 2024

Great article.

clifford davies - December 1, 2023

I set up a tank with a number of healthy plants. The tank remains completely clear for day after day. But as soon as I introduce a few fish the water immediately begins to cloud over. After about a week it is almost impossible to see the plants and the fish. Most frustrating and soul destroying.
Reducing the amount of light to just a few hours a day also failed to rid the tank of this pea soup quality.
Am now charging a certain amount of water weekly with a slight improvement.

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