Green Water in Fish Tank: Causes & Solutions

What is Green Water?

If you’ve had aquariums for any length of time, odds are you’ve had to deal with green water in a fish tank at least once. Green water is caused by a sudden population explosion of suspended algae known as phytoplankton. Any healthy tank will have some algae in it, but they usually grow on the glass or objects in your aquarium. Green algae are different though, as they float freely through the water, and can multiply very rapidly. This is known as a “bloom”. Once the number of microscopic algae becomes too high, the water will start to appear green, and eventually obscure the visibility in your tank. If left too long, the water will eventually become a murky greeny-yellow, very similar to pea soup.

Algae bloom can happen rather quickly too; one day your tank is crystal clear, and the next day you can’t see more than a couple of inches into the water. Water changes and filters may help temporarily, but to fix the problem once and for all, you need to get to the root of the issue. Solving the underlying issue will stop the green water algae from coming back over and over again.

Is Green Water Bad for Fish?

Although it can look unsightly, green water in your aquarium is not considered dangerous for fish. Many fish live in green-colored water in their natural environment, so they are naturally accustomed to it. However, if the algae multiply to the point that it reduces the amount of oxygen available, then it can become an issue for your fish. It is best to deal with it before it gets out of hand and affects the overall health of your aquarium. If you think that your fish are starting to struggle for oxygen, you can add a bubbler to help oxygenate the water while you are taking care of the issue.

green water
Green Water in Fish Tank

Causes of Green Water in Fish Tanks

Green aquarium water is the result of two existing conditions – the aquarium having too much light, and the water having a high concentration of nutrients. Both of these conditions are necessary for algae growth, and neither alone is sufficient.

Excess Nutrients

Algae thrives on nutrients such as nitrates and phosphates, so if you have too much in your water, this can cause an algae bloom. There are several things that can cause a spike in nutrients in your tank.

Overfeeding your fish, or having too many fish for your tank size is a common source of excess nutrients. Leftover food and fish waste are both sources of ammonia, which will cause a spike in nitrates and phosphates

Changing the water on your tank, and using nutrient-laden tap water to do it is another cause of excess nutrients in your tank.

If you have live plants in your aquarium, the fertilizer that you use for them will cause nutrient spikes as well. It is important to always follow the instructions regarding dosage and how often to use it, and to test the water in your tank after feeding your plants. If you aren’t sure about fertilizer dosage, it is best to err on the side of less rather than more, in particular, if you don’t use c0² or don’t have a lot of plants. Keep in mind when adding fertilizers that any excess nutrients will be used by algae to grow, as they are plants too.

(When working out doses, remember that an aquarium never holds as much water as it is designed to. Example – a 20-gallon aquarium usually holds around 16 gallons of water after taking into consideration the gravel, rocks, and any other decorations. This needs to be taken into consideration when working out dose sizes and concentrations)

Lack of Maintenance

A very common cause of fish tanks turning green is poor maintenance habits. Lack of water changes (and water tests) will lead to a gradual deterioration of water quality. Over time this will create an ideal environment for algae to grow and thrive. Aim to replace at least 20% of the water in your aquarium weekly, with a good deep cleaning every so often. Water should be tested quite frequently, as this can often be the only way to spot a potential issue before it becomes a problem.

A gravel vacuum should be used once a week, or every other week at most to remove all excess food and fish feces. Remember to remove all decor before starting, as a lot of waste will settle under and around the edges of them.

Hang-on filter cartridges should be replaced monthly, and canister-style filters should be serviced and cleaned regularly. Mechanical filter media should be rinsed or replaced every 4 to 6 weeks, the same as active carbon or any other chemical media.

If you have a fish die, or any dead plant matter, it should be removed from the tank as soon as possible. Allowing either to stay in your aquarium will cause unnecessary ammonia to be released into the water.

Too Much Light

Bright aquarium lights or exposure to direct sunlight can make your tank vulnerable to algae blooms. Algae thrives on bright light, as they are dependent on photosynthesis to survive and multiply. Leaving your tank lights on for too long per day can cause algae levels to increase.

How to Fix Green Water in Your Fish Tank

There are many ways to treat green cloudy water in your fish tank, depending on the level of algae in it. Some are methods you can start trying right away, and others are items that need to be purchased. Keep in mind that not all solutions will be forever, and no matter which you choose you’ll still need to monitor your water quality to avoid future issues.

Stop Overfeeding Your Fish

Overfeeding your fish is an avoidable source of algae. If there isn’t excess food laying in the gravel in the tank, it is one less source of food for algae.

The best way to gauge how much to give your fish is to only feed them the amount that they can eat in 2 minutes, once or twice a day. Many veteran aquarists will skip a feeding at least one day per week.

Lower the Stocking Level of Fish

Having the correct number of fish for your tank size is a good way to avoid algae issues. Too many fish per gallon is another avoidable source of nitrates and phosphates, so it is important to have the correct balance. The best ratio to maintain is 1 inch of adult fish per net gallon of water.

Add Live Plants

Live aquatic plants use the same nutrients as the algae that cause green water, so they are a very effective way of preventing algae blooms. They are a simple way of avoiding nutrient spikes, and they can make a great visual addition to any tank.

Floating plants such as hornwort, duckweed, and others can help reduce the amount of light that penetrates into the water, helping to reduce algae growth that way too. Most aquariums that have healthy plants will never experience any algae problems.

Add Live Daphnia

Live Daphnia (or water fleas) are another effective way to maintain healthy algae levels. Daphnia are tiny, freshwater crustaceans that feed on phytoplankton. The added benefit of daphnia is that they are an added food source for your fish, and also a great source of vitamins A and D for aquarium fish.

Lower Light Intensity or Duration

Choosing a light that is suitable for the size and depth of your aquarium is important. Aquariums that don’t have live plants in them don’t need really powerful lights.

Avoid placing your tank in a window that receives direct sunlight. If it can’t be avoided, install a background material to cut down on the light coming through the glass. Adding shades or curtains to eliminate as much natural light from the tank is another good option. If your tank is in a bright room, but not in direct sunlight, keep your tank light off during the day.

How long your lights are on each day is important too. Installing a timer is the best way to have your lights go on and off at set times each day, without having to remember or have alarms on your phone going off daily. Aquariums with plants require 8 to 12 hours of light daily, while tanks without plants can get by on 6 hours or less.

Eliminate Light

Since light is a trigger for algae blooms, removing the light altogether is an effective method of treating an outbreak.

The simplest way to do this is to wrap your aquarium in black plastic, black garbage bags, or dark blankets for a few days. Only take the cover off long enough to feed your fish, and then put the cover back on. In many cases, the algae bloom will die off within a few days, although you will still need to figure out what caused it in the first place. If you don’t see results within the first 48-72 hours, try another method.

If you have live plants in your aquarium, they should be able to survive for at least a week with no light, much longer than the algae will be able to survive.

UV Sterilizers

The most effective and effortless way to take care of a green water issue is to install an Ultraviolet (UV) Sterilizer on your aquarium. Sterilizers such as the Green Killing Machine work by passing water through a UV chamber. Suspended algae are eliminated, along with many other disease-causing organisms. Once installed, your aquarium will have crystal clear water again within a few days, with minimal effort. UV sterilizers are completely safe for fish, invertebrates, and plants.

Despite being relatively expensive not too long ago, UV sterilizers have come down considerably in price in recent years. They are now increasingly affordable, and also very easy to install.

With a UV sterilizer installed on your aquarium, you should never have a green water problem again.

Mechanical Filter with a Fine Filter Media

Mechanical filters are an effective method to treat green water, but they aren’t quite as effortless as a UV sterilizer. Filtering pads such as Polishing Filter Pad are effective at trapping and removing algae particles.

If you are thinking about installing this type of filter, just keep in mind it is not recommended for long-term everyday use.

Chemical Treatments

There are many different products on the market that are effective at getting rid of algae blooms. Before using chemicals, it should be noted that they should be a last resort in treating your green water issue, for a couple of reasons. First, not all are safe for certain plants, or invertebrates, and you can never really be sure of their effect on fish until it is already in the water. Secondly, they don’t actually address the cause of the problem, they just take care of the algae issue.

There are many products developed specifically to be used in your aquarium, such as Aqueon Algae Remover or API ALGAEFIX.

If you do choose to use chemicals to treat your green water issue, just make sure to read the label to make sure it is safe for everything that you have living in your aquarium.

As with most aquarium issues, green algae bloom can be prevented with some basic aquarium care, and a little bit of knowledge. If you find yourself dealing with an algae bloom, deal with treating the algae first and foremost, to make sure that your fish are healthy. Then you can start to look into the root cause of the outbreak, fix the issue(s), and then make sure they don’t happen again. With a little bit of knowledge, anyone should be capable of dealing with a green water issue in their aquarium.