Solving Green Aquarium Water: Know The Cause And Solutions!

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It can be a shocking and unnerving experience to walk past your beloved fish tank and find that the water has turned green. Not only is green water unsightly, but you’re also left wondering how it happened, and what it means for your fish. Feeling concerned is natural, but we’re here to tell you not to panic!

With a little investigating, you’ll soon figure out what turned your water green. And with minimal to no effort, you’ll be able to get your tank running smoothly once again! This article explains why your once crystal-clear water has changed color and what you can do to fix it.

What Causes Green Aquarium Water?

Simply put, green aquarium water is caused by algae blooms. Most aquarists may already be familiar with the term, but let’s take a closer look at what it means. 

FREE dirty overgrown seaweed and seashell glass in the aquarium

Algae is a catchall term for a type of simple, photosynthetic, and mostly aquatic plants that lack stems or roots. There are over 70,000 types of algae, and many of them occur naturally in freshwater tanks. Phytoplankton is a type of algae that is responsible for turning tank water green.

A controlled population of phytoplankton is harmless and even beneficial to your tank. In fact, many hobbyists even add phytoplankton cultures to their tank! However, certain conditions can trigger them to multiply exponentially, causing a free-floating algae bloom. Large quantities of suspended algae lowers the transparency of aquarium water and causes it to appear green.

Why Do Algae Blooms Happen?

Now that we’ve established that green aquarium water is caused by an algal bloom, let’s delve into why your tank is turning into the color of pea soup. These factors provide a conducive environment for phytoplankton to multiply, which results in green-colored water.

This phenomenon initially poses no harm to your fish, but will wreak significant havoc if left untreated – making this more than just an aesthetic issue.

The initial surge of phytoplankton will likely not cause an issue if you remedy it immediately. However, once they die, they become food for decomposing bacteria, who will use up oxygen in the tank for this purpose. This will in turn lower oxygen levels in your tank and harm your fish.

Given the potentially harmful implications of algae blooms, aquarium owners are often curious about how to prevent them in the first place. Let’s take a look at the two key reasons algae blooms happen:

Excess Light 

Well-lighted aquarium.

The number one cause of regular water turning green is excess UV light – either from aquarium lights, or from direct sunlight. Too much exposure to light can cause phytoplankton to multiply exponentially in your tank, as they can now perform photosynthesis at a greater rate.

Photosynthesis provides the suspended algae in your tank with more nutrients, allowing them to grow and reproduce more quickly. When they are present in large quantities in your tank, they turn your aquarium water green. 

If you’ve determined that sunlight is the cause of your problems, consider repositioning your tank so that it does not receive too much sunlight in the daytime. And if your tank light is the culprit, consider using a timer so that your tank only receives light during a specific period of time. Being able to control how much light your tank receives will minimize the risks of green water outbreaks.

Too Many Nutrients

Too much of a good thing can indeed be a bad thing. Most plants rely on nitrogen and phosphorus for their nutritional needs, so these compounds are beneficial in moderate quantities.

However, when nitrate and phosphate levels are too high, they create conditions that cause algae to grow excessively, resulting in green water.

These nutrients typically originate from fish waste and excess food, but they can sometimes occur as a result of nutrient-laden tap water. Therefore, testing is key. Perform regular water tests and to ensure that phosphate and nitrate levels are normal.

In addition, regular tank maintenance can prevent fish waste and uneaten food from building up. Performing a water change of 15-20% once in two weeks can go a long way!

How To Fix Green Aquarium Water

Proper maintenance lies at the heart of every green water treatment. Because the root cause of this issue is excess algae, fixing the problem has a lot to do with making sure that tank parameters and lighting levels are as close to optimum as possible.

Simply put, the goal is to correct excessive levels of light, nitrates and phosphates. Here are a few things you can do to a greenish, murky mess into crystal-clear aquarium water!

Keep Your Filtration System Running Smoothly

Your filter is your best friend in the fight against green water algae as it can keep nitrate and phosphate levels under control. Most filters contain activated carbon which neutralizes dissolved organics, ridding your tank of the organics which cause algae to bloom.

This all sounds amazing, but odds are, most tanks that turn green already have a filter. So what gives? Well, for one, you need a clean filter for it to work. This means changing filter cartridges monthly, scheduling filter cleanings monthly, and making sure you perform regular filter maintenance.

Choosing the Right Filter

The second thing you need to do is make sure that you have the right filter type for your tank. Power filters and canister filters are most commonly used, so it can be helpful to weigh your options between the two by comparing their differences. However, diatom filters are especially worth considering if you have a green water problem, as they are capable of removing diatomic algae from the system.

Last but not least, you need to get a filter with the right capacity. Choosing the right filter capacity for large tanks is especially important because a filter that is too small will not have the power it needs to function effectively. Meanwhile, if your filter is too strong for your tank size, you risk creating water currents that are too strong for your fish, creating a less-than-ideal habitat.

In short, keeping your filtration system running smoothly is a matter of maintenance, as well as choosing the right size and type of filter for your tank. Check these boxes, and you’ll find yourself with clearer water in no time!

Consider Using An UV Sterilizer

What exactly is an ultra-violet sterilizer? UV sterilizers are popular among aquarists because they effectively rid the water of free-floating microorganisms such as parasites, bacteria and algae. These sterilizers come equipped with a UV tube, through which water passes through.

UV sterilizers can help with green aquarium water as it kills free-floating algae by damaging their DNA. This means that most of the phytoplankton causing green aquarium water will die when they come into contact with UV rays, making this an effective way to rid your tank of this problem.

SunSun HW-304B

Are UV sterilizers safe?

Despite the many benefits of this nifty piece of equipment, hobbyists may have several concerns. Firstly, they may wonder if beneficial bacteria will be killed in the process. After all, if UV rays damage the DNA of single-celled microorganisms, wouldn’t it also damage the bacteria that form the backbone of their tank’s biological filtration system?

The good news is, most good bacteria aren’t free-floating. They are typically found along tank walls, among the substrate, or on the filters and sponges. While a small number of free-floating beneficial bacteria might be harmed, its impact on the overall population will be next to nothing as the vast majority will be attached to tank surfaces.

Another thing to note is that UV sterilizers are perfectly safe for fish and invertebrates. However, aquarium owners will want to be very careful as they handle this equipment, as UV rays can cause damage to their eyes and skin. Always refrain from looking directly at the tube, and be very careful as you are installing the equipment.

Chemical Treatments May Help

As a last resort, you may want to turn to chemical products for help. Chemicals may fix your greenish water, but it is important to note that they do not address the root cause of the problem. In addition, they may have unintended side effects on your fish, plants, and water. However, they are typically an effective solution when you are in a pinch.

The most common type of chemical treatment used to treat green water are algicides. These work by killing the algae in your tank. However, as previously mentioned, they are an effective, but short-term fix. Without addressing the underlying issues that caused algal blooms in the first place, it is very likely that they will occur again. 

Does Chemical Filtration Count?

When chemical treatments are referenced, they typically don’t include chemical filtration methods as part of the definition. Unlike chemical treatments, chemical filtration simply relies on chemicals to remove components that cause algae growth, such as phosphates and nitrates. Activated carbon and phosphate removers are commonly used in chemical filtration methods.

Meanwhile, chemical treatments include chemicals that are specially formulated to kill algae. While effective, it is a type of poison and may have unintended consequences on the health of your tank and its inhabitants. Therefore, it is important to dose accurately based on the volume of your tank, and avoid making it a go-to solution.

Chemical filtration and chemical treatments are typically used separately. Before administering any form of chemical treatment to your tank, be sure to remove all chemical filtration components.


Is Green Water Bad For Aquarium?

Yes, green water is bad for your aquarium. When water transparency is low, aquatic plants are unable to get the sunlight they need to perform photosynthesis. This will affect the health of your aquatic plants.

In addition, the presence of algae may reduce the amount of dissolved oxygen in your tank. This happens in two ways. Firstly, algae will carry out aerobic respiration at night, which requires oxygen. This makes oxygen more scarce at night. Secondly, algae that die will be decomposed by bacteria that use up oxygen in the process.

Both these factors reduce the oxygen concentration in your tank. Fish that do not have sufficient access to oxygen may suffocate to death in severe cases, or suffer from stunted growth in mild instances. Either way, fish keepers should do all they can to prevent algae blooms in their tanks.

Will Vinegar Kill Algae In Fish Tank?

While vinegar can be used to remove algae from the tank, they do not do so by killing the algae.

baking soda with brush and vinegar and a toothbrush

Vinegar is typically used to wipe down the surfaces of fish tanks as they are natural and fish-safe. Since conventional cleaners such as soap can’t be used on fish tanks, vinegar is seen as the perfect substitute. This means that a lot of the time, vinegar is used to remove algae growth from tank walls and plants.

Unfortunately, this is not a feasible solution for green tank water, which are caused by free-floating algae instead of algae that are attached to surfaces. Simply dosing your tank with vinegar will not fix the problem.

How Long Does It Take for UV to Clear Green Water?

The answer to this question depends on the size and strength of your sterilizer. However, most of the free-floating algae causing green water should be removed in under 48 hours.

Is Green Algae Good for Fish Tanks?

In small, controlled quantities, green algae is actually good for fish tanks as they can be a reliable food source for many underwater sea creatures. However, they are harmful in large, uncontrolled quantities as they will lower the oxygen concentration of your tank.


If you find yourself with a tankful of greenish water, it can be natural to panic and scramble for a solution. Frankly speaking, however, panicking doesn’t help the situation. Green water is caused by two key factors – light and nutrients, which are relatively easy to fix. As long as you remain methodical in your approach, you should be able to fix the situation in next to no time!

Did you enjoy this article? If you did, don’t forget to share it with a friend who will benefit from the insights outlined here! Have you ever dealt with a green water outbreak yourself? We’d also love to hear from you! Just talk to us in the comments below!

Wanda is a second-generation aquarist from the sunny tropics of Malaysia. She has been helping her father with his freshwater tanks since she was a toddler, and has fallen in love with the hobby ever since. A perpetual nomad, Wanda does her best to integrate fish-keeping with her lifestyle, and has taken care of fish in three different continents. She loves how it provides a nice break from the hustle and bustle of life.

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