How To Get Rid Of Green Water In Fish Tank

Green and cloudy water is one of the most common issues aquarium owners face. While some algae growth is normal, excessive green water isn’t just an eyesore – it can also be dangerous for your fish. When an algae bloom gets out of control, it reduces oxygen levels and releases toxins into the tank. Taking steps to eliminate green water is crucial for maintaining a healthy environment for your fish.

In this comprehensive guide, we’ll discuss the causes of green aquarium water and the best ways to clear it up. You’ll learn how to diagnose the root of the problem, whether it’s too much light, an imbalance of nutrients, or inadequate filtration. We’ll also cover effective solutions ranging from chemical clarifiers to UV sterilizers. Finally, we’ll look at steps you can take to prevent green water from recurring. With the right approach, you can get your tank looking crystal clear again.

green water in fish tank

Why Does My Fish Tank Water Turn Green?

Green water in an aquarium is caused by an explosion of microscopic algae suspended in the water, known as an algae bloom. While normal algae levels help absorb toxins and provide food for some fish, when algae grows uncontrolled, it causes problems. So what triggers excessive algal growth? There are a few common culprits:

Too Much Light

If your fish tank gets direct sunlight or very bright artificial light, it can enable rapid algae growth. Exposure to light fuels photosynthesis in algae, allowing populations to boom. Excessive duration of light exposure, typically more than 12 hours per day, is particularly problematic.

Excess Nutrients

Algae feeds on nitrogen and phosphates in the water, which come from fish waste and uneaten food. When levels get too high, algae has plenty of “fertilizer” to increase. Phosphates are a particular concern as filters do not remove them.

Poor Water Circulation

When water movement is inadequate, algae can multiply rapidly. Stagnant water allows algae to remain suspended rather than getting flushed out or removed by the filter. Poor circulation also leads to low oxygen in parts of the tank.

High Temperature

Warmer water speeds up the metabolism and reproduction of algae. Tanks with temperatures in the higher end of fish’s comfortable range are more vulnerable to algal overgrowth. Slow-growing green water algae also prefer warmer conditions.

New Tank Syndrome

Brand-new tanks often experience green water blooms during cycling as ammonia and nitrates spike while the biological filter is established. Once the tank matures, algae growth should stabilize.

Understanding the Impact of Green Water on Fish

While algae blooms look unpleasant, they also seriously impact fish health. Here are some of the main risks:


The number one problem green water causes for fish is stress. The cloudy water alarms fish and disrupts their environment. Continued stress weakens the immune system and makes fish prone to disease.

Reduced Oxygen

With dense algae blocking light penetration, oxygen-producing plants and beneficial bacteria cannot perform photosynthesis. Algae also competes for existing oxygen at night. Lower oxygen levels lead to labored breathing in fish.


Decaying algae releases toxins into the water, including ammonia and nitrites. At high concentrations, these compounds can poison fish. Excessive ammonia burns gills and reduces oxygen uptake.


Some species of algae produce compounds that irritate fish gills and skin. Fish may show signs of damage or exhibit flashing and rubbing behaviors to alleviate discomfort.

Reduced Visibility

Thick green water blocks light and reduces water clarity. Many fish become stressed when visibility drops below 6-12 inches. They may stop eating and become more susceptible to bullying and fighting.

Effective Methods to Eliminate Green Water in a Fish Pond

If your pond or aquarium is dealing with green water, taking quick action is key before algae can reproduce. Here are the most effective treatment methods to clear up green pond/tank water:

Chemical Clarifiers

Algaecide chemicals containing benzalkonium chloride or polyquaternium immediately start killing suspended algae. This provides rapid green water relief, though dead algae may make water cloudier before clarifiers clump and remove the particles. Avoid copper clarifiers, which can build up to toxic levels in ponds/tanks.

UV Sterilizers

As water containing green algae flows through a UV sterilizer unit, ultraviolet radiation kills the algae spores. The filter then removes them. UV sterilizers provide continuous algae control without adding chemicals. Units sized appropriately for pond/tank volume offer the most efficient green water reduction.

Diatom Filters

These specialized mechanical filters use powdered diatomaceous earth to capture microscopic algae in the water as it passes through. They filter algae 5-15 times smaller than most filters trap and remove green water instantly. Diatom filters must be cleaned frequently to sustain effectiveness.

Ozone Generators

Ozone oxidizes algae in the water as it diffuses through, turning green water clear. The amount of ozone needed to eliminate algae falls within safe levels for fish and plants. However, some substrates like silicone can be damaged by ozone over time. Generators require proper sizing and contact time with water for best results.

Blackout Method

Blocking all light from entering a tank for 3-4 days prevents algae from photosynthesizing, steadily decreasing the population. For best results, also cease feeding to avoid further nutrient additions. Once algae has substantially declined, a partial water change helps remove remaining algae. Light duration should then be reduced to prevent recurrence. This method is not effective for outdoor ponds.

How to Resolve Green Water Problems in a Fish Tank Swiftly

Green water issues often appear and multiply with alarming speed. Here are some approaches to get the situation under control quickly before algae takes over your tank completely:

  • Large and frequent water changes – Changing out 50% or more of the water immediately removes a portion of suspended algae. Do daily changes if needed to stay on top of the bloom.
  • Use a clarifier – Chemical clarifiers rapidly start killing algae and clump particles for removal by the filter. Effects are noticeable within hours.
  • Add activated carbon – The absorbent carbon soaks up dissolved organics contributing to algae growth. It also removes green water toxins for clearer, safer water.
  • Clean or replace filter media – Removing old media eliminates trapped organics while a new mechanical filter sponge clears more green water with each pass.
  • Limit light duration – Cover the tank or turn off lights to 8 hours daily or less to slow algae growth. This gives you time to address the root cause.
  • Add floating plants – They block light penetration while competing with algae for nutrients and oxygen, inhibiting bloom spread.
  • Use an algaecide designated safe for tanks – When green water persists, an algaecide containing polyquaternium or simazine (for fish only) can wipe out resistant algae.
  • Do a 3-4 day blackout – For severe blooms, blocking all light stops algae growth in its tracks so populations subsequently crash. Follow with a major water change before cautiously resuming lighting.

Struggling to Remove Green Water in My Fish Tank

Despite your efforts, green water algae can be stubborn to eliminate from tanks. Here are some common mistakes people make that allow green water to stick around:

  • Overfeeding – Excess food leaves uneaten nutrients algae rely on. Feed only what fish fully consume within a few minutes 1-2 times daily.
  • Inadequate Filtration – Filters that can’t handle the tank’s biological load or turnover water quickly enough enable algae growth. Upgrade to a higher capacity filter suited to the tank/pond size.
  • Direct Sunlight – Tanks before bright windows receive too much intense light. Use shades/curtains to diffuse and reduce light exposure to 6-8 hours daily.
  • Old Bulbs – Fluorescent tubes over 6-12 months old emit light spectra that favor algal growth. Replace bulbs annually for optimal fish viewing and to discourage green water.
  • Neglected Cleaning – Letting debris accumulate provides nutrients and raises dissolved organic compounds, which algae thrive on. Clean surfaces and change water weekly.
  • No UV Sterilizer – These units kill suspended algae daily, keeping populations in check. Add one size at least twice your tank’s volume for green water prevention.

Common Reasons for Green and Cloudy Fish Tanks

Green water is one of the most frustrating issues in fishkeeping. While the specific causes vary, there are some common culprits behind cloudy green aquarium water:

  • Overcrowding – Too many fish overload the biofilter with ammonia/nitrates, fueling algae. Stock conservatively and do frequent water changes.
  • Old Tank Syndrome – As the tank ages, dissolved organics accumulate, promoting green water. Activated carbon removes these compounds.
  • Low Oxygen – Inadequate surface agitation reduces oxygenation, allowing more algae to proliferate. Increase water movement with powerheads, airstones, or waterfall filters.
  • High Phosphates – Phosphates in tap water or fish food cause excessive algae growth. Use reverse osmosis water or phosphate-absorbing media to reduce levels.
  • New Substrate – Freshly added gravel or sand releases ammonia, causing initial algae blooms. Do plenty of water changes to get past the break-in period.
  • High pH – Alkaline water enables green water algae to thrive. Keep pH under 7.6, lower for soft water fish.
  • Direct Sunlight – Tanks in bright natural light often deal with recurrent green water. Block or diffuse intense sunlight.
  • Insufficient Plants – Blooms spread rapidly without enough plants to compete with algae. Add fast-growing stem plants for natural algae control.

Dealing with Foul-Smelling Green Water in a Fish Tank

A fish tank with green water often develops an unpleasant rotten smell. This is typically caused by:

  • Decaying Organic Matter – The decaying biomass releases noxious compounds as green water algae die in large numbers. Removal by filters and water changes helps reduce the stench.
  • Poor Water Quality – Excess ammonia and hydrogen sulfide gas from rotting debris produces a foul odor. Improved tank maintenance and algae removal clears the smell.
  • Anaerobic Pockets – In areas of poor circulation, sulfur bacteria thrive, generating foul odors. Increase water movement and oxygenation to prevent this.

To eliminate foul odors accompanying green water, perform large water changes to dilute organics while adding activated carbon to filter out smelly molecules. Thoroughly clean the substrate to remove rotting debris and improve flow. Then, address the root algae problem with UV sterilization. The smell of cleaner, clearer water should dissipate within a day or two.

Green Water Issues in Aquariums – Discussion, Experiences and Expert Opinions

Green water is the bane of aquarists everywhere. Let’s discuss this problematic phenomenon and share tips on battling it in our tanks:


“I had green pea soup water that wouldn’t clear despite huge water changes. Added a UV sterilizer, and it was crystal clear in 48 hours finally. Now I use the UV full time to keep the green monster away.”

“My green water turned out to be dinoflagellates, not regular algae. Had to treat with chemicals to kill it off since UV and diatoms didn’t work.”

“Green dust algae is only algae that rapidly reappear after blackout treatment. Had to install window film to reduce sunlight intensity.”

Expert Opinions:

“Green water often indicates an imbalance of nutrients and light. The key is figuring out which factors are driving the bloom.”

“Manual removal of green water can seem futile, but UV sterilization provides reliable, ongoing control when sized properly.”

“Prevention is crucial. Limiting light exposure and fertilization along with adequate plant growth helps suppress algal blooms.”

“Green water algae take advantage of organics and ammonia to thrive. Keeping these reduced through gravel vacuuming and water changes is important.”

Rapid Green Water Formation in Fish Tanks – What Causes it and How to Slow it Down

Green water can form surprisingly quickly, sometimes turning crystal clear water into pea soup overnight. What contributes to such rapid blooms?

Factors that Accelerate Green Water Formation:

  • Direct Sunlight – Intense light enables algae to multiply exponentially faster. Use window tinting or reposition tank.
  • Excess Nutrients – Heavy feeding and fish waste creates optimal conditions for explosive algae growth.
  • High Temperatures – Warmer water accelerates algae reproduction. Cool water down closer to 72-76°F.
  • Poor Circulation – Stagnant areas allow concentrated algae accumulation. Improve flow and surface agitation.
  • New Tank Cycling – Ammonia and nitrates spike during the cycle, fueling heavy algae blooms. Patience and water changes help.

How to Slow Down Green Water Formation:

  • Black Out Periods – A few days of darkness stops photosynthesis, slowing algae growth dramatically.
  • Reduce Light Duration – Limit light period to 6-8 hours daily. Use timer to enforce consistency.
  • Add Floating Plants – They obstruct light penetration to algae and absorb nutrients, restricting growth.
  • UV Sterilization – UV radiation steadily kills algae each time water passes through, keeping numbers down.
  • Diatom Filter – Removes microscopic algae 5-15 times smaller than typical filters trap.
  • Clean Often – Frequently remove organics that feed algae growth through water changes and gravel cleaning.

Natural Solutions to Clear Green Water in a Fish Tank

For those wishing to avoid chemicals or UV units, nature offers some other options to defeat green water algae naturally:

  • Fast Growing Floating Plants – Hornwort, anacharis, water lettuce, and duckweed grow rapidly to absorb excess nutrients while shading out algae below. With reduced light and food, green water fades.
  • Algae Eating Fish – Plecostomus, otocinclus, and other algae grazing fish can help nibble away at green water. Best results come from a school of 5-6.
  • Beneficial Bacteria – Bacterial additives like Microbe-Lift consume organics and outcompete algae for nutrients. Restoring balance helps suppress blooms.
  • Phosphate Absorbers – Chemical media like phosphate sponges and pads soak up the phosphates in the water that feed algal growths.

Natural Predators for Green Water Algae:

Daphnia – These tiny crustaceans filter feed on single celled algae. A handful added to the tank can graze away green water.

  • Ramshorn Snails – Snails keep surfaces free of algal growth while feeding on green water. Avoid overpopulation.
  • Flagfish – Voracious algae eaters, flagfish can consume up to three times their weight daily in hair algae and microorganisms like green water.

Tips for Eliminating Green Algae on Aquarium Glass

While green water algae floats freely, eventually some may cling to the glass as an adhesive green film. Removing it improves visibility.

Use an algae scrubber or plastic card to scrub the glass and vigorously dislodge the attached algae. Focus on scrubbing rather than wiping which can just spread algae around.

For heavy growth, use a razor blade scraper. Carefully scrape against the glass at a 45-degree angle to remove clinging algae.

After scrubbing, use a gravel vacuum to remove all loosened debris from the tank. Siphon thoroughly around decorations, too.

Reduce light duration and direct sunlight exposure, which enables algae growth in the first place.

Consistently removing algae establishes conditions less favorable for regrowth. Persistence pays off for an algae-free aquarium.

The Connection Between Water Changes and Green Fish Tanks

There is an important relationship between water changes and managing green aquarium water. Here’s how proper water changes can help:

  • Removes Algae Directly – Changing out 30-50% of water manually extracts a portion of suspended algae each time. This works with both green water and green spot algae on glass.
  • Resets Nutrient Levels – By draining water containing fish waste and excess food, replacement water dilutes organics that drive explosive algae growth.
  • Improves Oxygenation – Stagnant water becomes oxygen depleted. Fresh water from changes improves oxygen for healthy bacterial colonies to outcompete algae.
  • Eliminates Toxins – Algae decay releases ammonia, nitrites and other compounds toxic at high levels. Water changes dilute these for safer water.

The right amount and frequency of water changes depends on stocking levels, growth of algae and your tap water parameters. Start with 30% weekly and adjust as needed to keep algae under control. Be sure to vacuum the substrate thoroughly when changing water as well.


Green water may be common in tanks, but luckily armed with the right information, you can get those unsightly blooms under control. By understanding what enables green algae to multiply out of control, such as excess light, nutrients and low oxygen, you can make adjustments to deter growth. Employing solutions ranging from beneficial bacteria and floating plants to clarifiers and UV sterilizers allows you to attack green water on multiple fronts. Putting some elbow grease on manual removal through large water changes accelerates improvement. While green water can be frustrating, persistence in applying the techniques discussed will regain your aquarium’s vibrant, crystal clear state. Maintaining optimal conditions and vigilance over algae growth will help keep the dreaded green at bay. Your fish will be healthier and happier, and you’ll be able to enjoy the underwater views again.