Why Is My Fish Tank Green? Causes and How to Clear It

The short answer

Green water is a bloom of tiny algae floating in the water itself. It often follows too much light plus extra nutrients from food, fish waste or tap water, and an ammonia spike can set it off. Water changes alone do not flush it out; a UV sterilizer or a blackout helps clear it.

Aqueon, an aquarium equipment maker, describes a tank turned to “pea soup” overnight, a big water change and a new filter cartridge, and then: “Things improve for a day or two, but then it’s back.” The fix that lasts depends on which trigger fed the bloom.

Green water is floating algae

Aqueon says green water outbreaks are “caused by a sudden population explosion of suspended algae known as phytoplankton”. Unlike algae on the glass or plants, these cells drift in the water and “multiply by the billions in a short period of time”.

Tinted water is green water. Clear water with a green film or spots on the glass, plants and decor is algae growing on surfaces, which our guide to why algae grows on aquarium plants covers. A green haze can also pass for ordinary cloudiness; our guide to why aquarium water turns cloudy separates a green algae bloom from a milky bacterial bloom.

What turns the water green

Aqueon says too much light “in the presence of a source of nutrients” can “send algae into hyperdrive”. Aquarium Co-Op, an aquarium retailer, adds an ammonia spike, and API, a maker of water treatments and test kits, notes that “even an established aquarium can go through the Nitrogen Cycle at any point”.

Trigger	Where it comes from	How to check
Too much light	A sunny window, the light on too long, or a light too strong for the tank (Aqueon)	Count the light hours; note any direct sun
Extra nutrients	Nitrate and phosphate from food, fish waste and tap water; too much fertilizer (Aqueon, Co-Op)	Test tank and tap water for nitrate and phosphate
Ammonia spike	A new tank not yet cycled, or overfeeding (Co-Op)	Test ammonia and nitrite
Too many fish, too little upkeep	Overstocking, overfeeding, skipped water changes (Aqueon, API)	Review stocking, feeding and water changes

Why a water change doesn’t clear it

Aquarium Co-Op says the algae replicate so quickly “that you cannot flush them out with large water changes”, and fine filter floss does not catch them either. In an r/Aquariums thread on Reddit, one owner wrote, “We just did about a 50% change a couple weeks ago, and it drifted back to that in about 4-5 days.”

The new water can also feed the bloom. Aqueon notes that “Performing water changes with nutrient-laden tap water will have the same effect” as overfeeding: a build-up of nutrients. Water changes still help after a fix: API says a partial change with a gravel siphon will “pull out and dilute the nutrients”.

Test before you treat

API’s first step for green water is to test, to “make sure the aquarium is not going through the Nitrogen cycle”. The results also point to the trigger.

Test ammonia and nitrite

Compare each result with your kit’s color card and, in API’s words, “follow the recommendations on the card if action is required”. Our guide to high ammonia in an aquarium covers the usual causes and how to bring it down.

Test nitrate

API’s Freshwater Master Test Kit covers “ammonia, pH, high range pH, nitrite and nitrate”. Aqueon advises testing regularly for nitrate and phosphate and correcting levels that are too high.

Test phosphate with its own kit

Phosphate is not on the master kit’s list, and API notes that “High Phosphates could be feeding algae growth”.

Test your tap water

Aqueon says “Tap water can contain high levels of nitrate and phosphate also, so it’s a good idea to test yours before using it in your aquarium”.

Warning

If your fish look unwell, test the water now and contact an aquatic veterinarian. Aqueon says green water algae are “typically not harmful to your fish”, but Aquarium Co-Op warns that an ammonia spike can harm fish. This guide covers the water, not fish health.

Ways to clear green water, side by side

Method	What the source says	Caution
UV sterilizer	“The most effective and effortless way” (Aqueon); kills the algae “within two to three days” (Co-Op)	Follow your unit’s manual; Evolution Aqua’s evoUV manual: “Avoid exposure of eyes and skin to ultraviolet radiation.”
Blackout	A few days, feeding daily (Aqueon); 7 to 10 days (Co-Op)	Plants may suffer; dead algae can spike ammonia
Algaecide	API ALGAEFIX is made to control “green water” algal blooms	A last resort (Aqueon); not with freshwater crustaceans, including shrimp, crabs, and lobsters (API)
Flocculant	API ACCU-CLEAR binds green water into clumps for the filter	Do not overdose (API)

How a UV sterilizer clears it

Aquarium Co-Op explains that a UV sterilizer slowly passes tank water past an ultraviolet bulb, and that “The UV changes the cell structure of the algae so that it can’t reproduce”; afterwards, “you can do multiple water changes over time to remove the green water”.

Green water is fairly easy to treat, Co-Op says, “so you don’t need to get a very big sterilizer”. For scale, Aqueon says its “QuietFlow UV Ultraviolet Sterilizer is designed for aquariums up to 40 gallons”, and that Coralife Turbo-Twist UV Sterilizers “are available in 3 sizes, rated for aquariums up to 500 gallons”.

Evolution Aqua, a UK maker of aquarium and pond filtration, says in its evoUV aquarium manual to “always combine with effective mechanical filtration to remove debris from the water and to capture dead and damaged algae cells”. For its units it says to “replace the UV bulb with new every six months”.

Safety

Evolution Aqua’s evoUV manual warns: “Warning: Ultraviolet radiation. Avoid exposure to direct or reflected radiation.” It also says “Disconnect the appliance from the supply before replacing the UV source”, “Do not operate the appliance if the protective enclosure or protective parts are removed or damaged” and “Avoid exposure of eyes and skin to ultraviolet radiation.” Follow the manual for your own unit.

A blackout, done carefully

Algae need light, so darkness can make the bloom die back, but the sources disagree on how long. Aqueon says to keep the tank covered “for a few days”: “If you don’t see results in 48 to 72 hours, another course of action is recommended.” Aquarium Co-Op, which itself prefers a UV sterilizer, describes “wrapping a blanket around the tank for 7 to 10 days”. The steps below check at 48 to 72 hours; the longer the tank stays dark, the more Co-Op’s cautions apply.

Turn the light off and cover the whole tank

Aqueon suggests black plastic, trash can liners or blankets. Co-Op warns that “Green water can survive off very little light, so make sure the aquarium is completely blacked out”, and that “your plants may suffer from the lack of light”.

Uncover only to feed

Aqueon: “Uncover the aquarium long enough to feed your fish each day, then cover it up again.”

Test ammonia while the tank is covered

Co-Op warns that “the dead algae can create an ammonia spike that harms the fish or causes another green water bloom”.

Check at 48 to 72 hours

If the water is not clearing, Aqueon recommends another course of action (see the table above).

Remove the dead algae, then fix the cause

Co-Op’s method ends with a large water change “to take out the dead algae”. Treat the new tap water with a water conditioner, as API’s conditioner label directs, and skip gravel vacuuming in a newly set up tank or a planted tank that uses substrate fertilizers, as Aqueon advises. Even when it works, Aqueon says “you will still need to address the cause of the bloom”.

Algaecides and clarifiers: the last resort

The products below are labeled for freshwater tanks, and this section covers freshwater aquariums only; for a saltwater or reef tank, follow your own product makers’ directions.

Aqueon, which sells its own Algae Remover, says “chemicals should always be a last resort”, because “they do not address the cause of the problem, and you can never be sure of their effects on fish, plants or the balance of your aquarium.” The makers’ own cautions:

Aqueon, for any chemical treatment: “remove activated carbon and other chemical filtration media from your filter, maintain good water circulation in the aquarium, and dose according to net gallonage”, which allows for water displaced by gravel and decorations.
API ALGAEFIX, which API describes for use in a “freshwater aquarium”: “It is a violation of federal law to use this product in a manner inconsistent with its labeling.” The label says “Do not overdose”, carries the caution “do not use with freshwater crustaceans, including shrimp, crabs, and lobsters”, and says to “Siphon or scrape any dead algae from aquarium.”
API ACCU-CLEAR, a flocculant rather than an algaecide: API says it binds green water into clumps that settle to be siphoned out or are caught by the filter pad. Its label also says “Do not overdose.” It is a freshwater clarifier: API says that in “marine (or saltwater) aquariums, API ACCU-CLEAR water clarifier will not clear cloudiness and could even make it worse.”

This guide gives no doses: follow the label for your product and your tank’s net volume.

Keeping the water clear afterward

Prevention is the triggers in reverse. From Aqueon’s advice:

Light: “Planted aquariums need 8 to 12 hours of light daily, while non-planted aquariums can get by on 6 hours or less.” Use a timer, and keep the tank out of a sunny window or draw the shades.
Feeding: “feed only what your fish can consume in 2 minutes or less, once or twice a day”.
Water changes: “10% weekly or 25% bi-weekly” (every two weeks) as a routine starting point, with the same gravel-vacuum exceptions as above. API says to use its conditioner “when adding or changing water”.
Plants: live plants use the same nutrients as algae, and floating plants such as duckweed and hornwort “help reduce the amount of light penetrating deeper into the water”.
Fertilizer: “err on the side of less vs. more”, especially without CO₂ or many plants.

Ready to buy?

Our freshwater aquarium test kits roundup covers testing, and our Filters & Media section gathers filters, including models with a built-in UV sterilizer.

Frequently Asked Questions

Q. Is green water bad for fish?

Aqueon says the algae are “typically not harmful to your fish”, and Aquarium Co-Op notes green water is even good for raising fry. The trigger behind it can be another matter, so test the water and contact an aquatic veterinarian if fish look unwell.

Q. Will green water go away on its own?

None of the sources promises that. API says patience and consistent maintenance help the water clear over time; the faster fixes are a UV sterilizer or a blackout.

Q. Why is my tank water green but there is no algae on the glass?

Because the algae are in the water. Aquarium Co-Op describes green water as free-floating, single-celled phytoplankton, so the glass can stay clean.

Q. Do LED lights cause green water?

Aqueon does not single out a type of light. It points to a light too strong for the tank or left on too long, and advises “a light suitable for the size and water depth of your aquarium” on a timer.

Q. Why is my fish tank going green on the glass?

That is algae growing on surfaces, not green water. For hard green spots, Aquarium Co-Op suggests “a razor blade (for glass tanks) or gift card (for acrylic tanks)”.

Sources (12)

Aquarium Tales Team

Written and reviewed by the Aquarium Tales Team. See the About page for how we work.