Green Aquarium Water – Causes, Prevention, & How To Clear It

    Green aquarium water

    Does your fish tank look a lot like pea soup? Have you ever wondered “Why is my fish tank green?” Green fish tank water is a common problem. But what causes green aquarium water in the first place?

    Green aquarium water is caused by blooms of algae. These microorganisms feed on light and suspended nutrients in your water. A sudden increase in either of these parameters is the most common trigger for their fast growth.

    Green water algae is unattractive. But with a few changes to your maintenance regimen, you will have crystal clear water again in no time!

    What Causes Aquarium Water To Turn Green?

    Green aquarium water is caused by single-celled free-floating algae organisms called phytoplankton.

    While these algae are not true plants they share many of the same characteristics.

    They use photosynthesis to create their own food from light.

    Algae also produce oxygen, which fish (and people) breathe.

    This video is an excellent visual breaks down of what green water algae are. It also covers many of the solutions we’ll be discussing below.

    YouTube player

    Suspended algae are different from green hair algae and other forms that attach to hard surfaces.

    In a pond or fish tank, it is easy to wipe away these forms. But algal growth that is suspended in the water is a different story.

    Unless you change all of the water in the tank, the green water problem can come back. Often at a shocking pace.

    So what causes green water in most aquariums and how do we correct it?

    Excessive Light

    Too much light is the number one cause of green aquarium water.

    Algae are photosynthetic organisms. The more light there is, the faster the algae bloom will happen.

    Direct sunlight hitting a fish tank is even more powerful than just leaving the lights running.

    Your tank light may not have the right output for plants and algae. Especially if you run incandescent bulbs.

    But natural sunshine has the ideal spectrum for photosynthesis.

    High Nutrient Levels

    Algae growth is fueled not only by light but also by nutrients.

    Usually, nitrate and phosphate levels have become too high.

    Nitrate is the end result of the nitrogen cycle in fish tanks.

    Starting with ammonia (NH3+), the beneficial bacteria in the system break that down into nitrite (NO2-).

    The nitrite is then broken down further into nitrate (NO3-). Nitrate does not go anywhere unless you have plants or algae to consume it.

    If you don’t keep up with water changes, nitrate levels rise, which encourages algae growth.

    Phosphates come from fish food. Most of it passes right through the bodies of the fish that eat it.

    When fish poop decays, phosphates enter the water column as algae fuel.

    Too Many Fish

    Another contributor to poor water quality is having too many fish.

    Fish release not only carbon dioxide but also ammonia as waste.

    Ammonia is even better than nitrate for algae fertilizer.

    And the more fish you have, the more you have to feed.

    Fish poop and leftover food further increase phosphate levels. These grow algae of all kinds.

    Is Green Aquarium Water Bad For Fish?

    Is green water bad for fishj
    Pearl gourami (Trichopodus leeri)

    Green aquarium water is not bad for fish. Many fish prefer the murky conditions that an algae bloom provides.

    Green water is darker, helping fish feel safe and less exposed.

    Green water algae metabolize carbon dioxide, releasing oxygen just like plants do.

    They also use ammonia, nitrite, and nitrate as fertilizer, keeping water conditions for fish safe.

    The smallest of fish fry will also eat these single-celled organisms.

    These free-floating algae are really an aesthetic issue.

    A green aquarium just does not look good. It looks soupy and unkempt.

    Plus, it’s a lot harder to see and enjoy your fish.

    How To Clear Green Aquarium Water (3 Different Step-By-Step Methods)

    How to clear green aquarium water
    Red discus (Symphysodon discus)

    The three best methods for fixing green water are using a UV sterilizer, algicide, and/or a diatom filter.

    All three do such a good job that I would recommend choosing one, rather than all at once.

    UV sterilizers are a bigger investment.

    But they are also more thorough in scrubbing a tank clean of green water algae.

    Aquarium algicide works in just a few days. However, they can be dangerous to live plants, depending on the brand.

    Diatom filters are also excellent. But their effectiveness depends on how well your filter turns over your tank. If your filter is not very powerful there will always be patches of water that don’t get filtered.

    Water can also bypass the filter pads in a weaker unit.

    Allowing green water algae to survive and continue to grow. Use a diatom filter cartridge inside of a canister filter for the best results!

    Supplies Needed

    Step-By-Step UV Sterilizer Removal Method

    UV sterilizers used to be too specialized for anyone but pond keepers and coral reef keepers.

    But in recent years they have become so compact and affordable that they make an ideal solution for green aquarium water.

    AA makes a fantastic UV sterilizer for aquariums up to 50 gallons in volume.

    The unit runs on its own pump as well. There are other sterilizer designs you can buy that attach to a secondary pump.

    Both do a good job of stopping an algae bloom.

    Step 1:

    Remove the unit from the box and go over the instructions to install the UV bulb.

    Step 2:

    Find a spot in the aquarium where the unit gets plenty of water flow.

    You also want it easy to access without being so visible that it detracts from the beauty of your tank.

    I recommend placing the UV sterilizer near the intake pipe of your filter or heater.

    Many aquarists prefer to hide submerged equipment using aquarium plants.

    If you do, make sure your plants don’t get sucked into the sterilizer’s intake.

    This can block the flow of water and even cause the pump to overheat and burn out.

    Step 3:

    Track the number of days the unit is running.

    UV sterilizers can kill an algae bloom in as little as a week.

    However, they do need to be allowed to run for the entire time.

    Step 4:

    Once your green water is gone, unplug the unit.

    UV sterilizers are indiscriminate killers.

    Anything that gets swept in is blasted by UV radiation, including beneficial bacteria.

    Step-By-Step Diatomaceous Earth Removal Method

    Diatom filter pads work by using diatomaceous earth.

    This powder is actually the dead skeletons of diatoms, another sort of planktonic organism.

    Diatom skeletons are so fine that they form a mesh that lets water pass through.

    But they screen out green water algae and other fine particles.

    Assuming your filter can hold custom cartridges, diatomaceous earth is great for cleaning up green water.

    Many filter designers (Whisper, Marineland, etc) build their units to only be compatible with their brand’s cartridges.

    Power filters by Aquaclear allow you to add any sort of media you wish!

    Step 1:

    Add your diatomaceous earth into a refillable filter media bag.

    You may need a custom-designed media bag, depending on the brand on your filter.

    Only add enough that the bag creates a barrier about a centimeter thick.

    You don’t want to overcrowd your filter as this can cause water to flow right past the media.

    Step 2:

    Rinse and hand-squeeze the bag of diatomaceous earth in a small amount of aquarium water.

    This pulls out any fine dust, which could irritate fish gills.

    Step 3:

    Add the diatom cartridge to your filter chamber. Allow it to run for 1 to 2 weeks.

    The pad will slowly take on a green color as the algae growth gets trapped within.

    Step-By-Step Aquarium Algicide Method

    Of the three, aquarium algicide is by far the simplest.

    Chemical treatment of greenish water will bring your crystal clear water back in just a few days.

    The only downside is that if you have especially heavy algae growth, the algicide can cause more problems.

    Once all of that algae is dead, it will start to decay.

    Bacteria will break it down into ammonia and other pollutants while also consuming oxygen to fuel their growth.

    If your algae all die at once, ammonia levels can shoot up. UV sterilizers and diatom filters are slower working.

    This is an advantage since your water quality won’t be impacted by them.

    So be ready to perform large water changes when using any chemical treatment for green water algae.

    Step 1:

    Test your water conditions. If ammonia or other nitrogenous waste levels look high, do a water change.

    Then test again.

    We don’t want the fish tank to crash from the sudden algae death.

    Step 2:

    Add algicide in a dose according to the instructions on the bottle.

    For API Algaefix, that is 1mL per 10 gallons of aquarium volume.

    Step 3:

    Re-dose as the brand recommends.

    API Algaefix is dosed every 3 days.

    I also recommend doing water tests on a daily basis using a Freshwater Master Test Kit.

    If ammonia levels spike, perform a water change.

    And then add enough algicide to bring concentrations back to the recommended level.

    Step 3.5:

    Monitor the leaves and color of any live plants you have.

    Holes, loss of color, and browning are signs that the algicide may be killing them.

    Many brands claim to be safe for a planted tank. But aquatic plants differ a lot in their response to chemicals.

    In my experience, slow-growing rosette plants like java fern and Bucephalandra sp. are harmed by algicides.

    Fast-growing stem plants like water sprite, hornwort, and moneywort are safer.

    How Long Does It Take To Clear Green Aquarium Water?

    Severum cichlid (Cichlasoma severum)

    How long your green water algae bloom takes to go away depends on the method you choose. If you decide to use a chemical algicide, it will take 3 to 5 days, on average.

    A UV sterilizer can take up to a week to clear your water.

    It depends on the size of your tank, the strength of the pump, and the sterilizer itself.

    Diatomaceous earth filter media also takes about a week.

    The design and flow strength of your filter will greatly affect how well this method works.

    How To Prevent Aquarium Water From Turning Green

    Dwarf Puffer (Carinotetraodon travancoricus)

    Preventing green aquarium water is a good idea not only because of algae.

    The conditions that encourage its growth tend to create other problems for hobbyists.

    Lower Your Light Levels

    Leaving the aquarium light running for too long is a recipe for algae of all kinds to thrive.

    All plants love light, including algae.

    If you have a plant-free aquarium, a week-long blackout may be enough to completely kill your green water.

    Make sure to do water tests daily in case of an ammonia spike.

    A less extreme measure is to use an aquarium light timer.

    Nicrew makes timer units that are compatible not only with their own (excellent) lights, but also other brands.

    Set the timer to run aquarium lights for just a few hours a day around feeding time.

    The fewer the hours of run time the better.

    2-3 hours in the morning, an afternoon blackout, and 2-3 hours in the evening is a good start.

    Adjust as needed based on how the green water responds.

    Avoid direct sunlight exposure to your aquarium.

    The sun’s spectrum is ideal for photosynthesis.

    Sunlight also carries a lot of heat, which can warm your water temperature dangerously fast.

    Last, consider if your light is too intense for your fish tank.

    Lights that are too intense are a common problem for smaller aquariums.

    If you can, adjust either the intensity knob.

    Or the height of the light so it shines from further up.

    Keep Fewer Aquarium Fish

    An understocked tank is a healthy tank.

    I always recommend adding fewer fish than the maximum capacity of a tank allows.

    Fewer fish means more space for the pets you do have.

    It also means less ammonia, fish poop, leftover food, and other nutrients for algae.

    If you really want a lot of fish, stick to smaller species.

    Small fish have much less mass for their length than larger fish do.

    That’s why the old one-inch-per-gallon rule does not work with anything but small fish.

    Ten 1-inch neon tetras do not equal one 10-inch oscar cichlid, in terms of their impact on an aquarium’s carrying capacity!

    Don’t Overfeed

    When you see an algae bloom in nature it’s almost always a sign that you’re looking at a nutrient-heavy environment.

    Nitrate and phosphate levels from farm runoff cause green water in lakes and streams.

    And in a home aquarium these same nutrients fuel algae growth.

    So take a moment to read the ingredients label.

    Food with high phosphate levels should be avoided if you have no live plants.

    Nitrate and other nitrogenous wastes come from leftover food that decays.

    As an important algae nutrient, it gets removed through regular water changes.

    If you accidentally overfeed, don’t leave food in the tank to rot.

    Rotting food is also a cause of white algae.

    Use your gravel vacuum to do a small water change right then!

    Add Live Aquarium Plants

    Nothing helps keep algae under control like adding some competition!

    Aquarium plants do this not only by consuming the same nutrients that algae do.

    Plants also reduce light entering the water column.

    There are hundreds of plants you could add to your aquarium.

    If you just want algae control, free-floating plants are the easiest to keep and do the best job.

    Submerged plants are not as effective at using trace nutrients and low levels of carbon dioxide.

    Algae are simpler organisms and grow faster.

    Floating plants can get all the light and CO2 they need from the surface.

    This, plus their ability to create heavy shade, makes them the perfect counter for algae.

    A few excellent surface plants for beginners are water lilies, red root floater, duckweed, and hornwort.

    That said, planted aquarium keeping can be about much more than just algae control.

    A few new plants may inspire you to delve deeply into the world of aquatic gardening!

    Green Water Algae Eaters

    Believe it or not, there are a few freshwater algae eaters that do eat green water algae.

    The first is freshwater clams and mussels!

    These filter-feeding mollusks are planktivores.

    This is why they die in 99% of fish tanks: the water is too clean for them.

    But green water algae is a rich soup of food for a clam.

    One clam per 5 gallons will reduce or eliminate green aquarium water for you.

    Clams are challenging to keep long-term, though.

    Once the algae disappears the clams will starve.

    They are also highly sensitive to even low levels of ammonia and nitrite.

    The second common filter feeder is water fleas (daphnia).

    These are tiny crustaceans that float in the water column.

    Daphnia can be bought in large numbers and will consume an algae build-up in days.

    They are also a great food for aquarium fish.

    So add an overwhelming amount (hundreds or thousands) at a time.

    That way, your fish won’t be able to eat them all right away.

    You also need to unplug the filter since water fleas will be sucked up by them.

    These downsides make water fleas only feasible for small tanks with low fish populations.

    Or unfiltered planted tanks (like Walstad aquariums).

    FAQs About Green Aquarium Water

    Green aquariums water

    Why Is My Fish Tank Water Green?

    Green water in a fish tank is usually caused by an excess of algae growth.

    Factors like excessive light, high nutrient levels, and poor filtration can contribute to this issue.

    To combat green water, you can reduce lighting, perform regular water changes, and consider adding live plants or UV sterilizers to control algae growth.

    How To Fix Green Aquarium Water?

    To fix green aquarium water, start by checking the water parameters and adjusting them if necessary.

    Perform regular water changes and clean the tank, including filters and decorations.

    Use a UV sterilizer or algae control products to prevent and remove green algae growth.

    Proper lighting and reducing excess nutrients can also help in resolving the issue.

    Why Does My Fish Tank Turn Green So Fast?

    There are several reasons why your fish tank may turn green quickly.

    One common cause is an excess of algae growth due to high levels of sunlight or overfeeding.

    Insufficient cleaning and maintenance can also contribute to this issue.

    Regular water changes, reduced sunlight exposure, and proper feeding can help prevent rapid green discoloration in your fish tank.

    Why Is My Fish Tank Green After A Water Change?

    A green fish tank after a water change may indicate an algae bloom due to excessive light or nutrient imbalance.

    Algae growth thrives in an environment with high levels of nutrients and light exposure.

    Consider reducing the lighting duration, adding live plants, and adjusting nutrient levels to restore water clarity.

    Regular maintenance and proper balance will help prevent green water in your fish tank.

    How To Stop Fish Tank Going Green?

    To prevent your fish tank from turning green, ensure you don’t overfeed your fish and clean the tank regularly by removing debris and performing partial water changes.

    Avoid direct sunlight and consider installing a UV sterilizer or adding live plants to maintain a healthy balance in the tank.


    Green water algae is caused by excessive light, especially direct sunlight.

    A high concentration of nutrients in your aquarium water is another contributing factor to green aquarium water.

    It isn’t dangerous for fish but it is unattractive.

    Fortunately, you have several control and prevention strategies here that will help clarify your water quickly!

    Recommended Reading:


    Don’t miss out on valuable tips! Subscribe below for our newsletter and get weekly updates on newly published posts.

      We respect your privacy. Unsubscribe at any time.