Why Is My Fish Tank Cloudy? (With Quick Solutions)

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Almost every aquarist will experience cloudy water in their fish tank at some point. This issue can be quite discouraging, as you can hardly see the inhabitants of the tank or its decorations.

From my experience, this topic can be split into two common scenarios:

  • A tank with white cloudy water.
  • A tank with green cloudy water.

So, in this article, I will list the most common reasons for each scenario and provide some practical and quick steps to fix the problem. Let’s dive right into it.

White Cloudy Water In Fish Tanks

Cloudy aquarium water in a saltwater tank.

Cloudy water is not always a sign of trouble. Ultimately, your response will depend on the severity of the issue. 

With some tanks, you can barely see the milky texture. In others, the water is so cloudy that it obscures the fish. 

You have two primary culprits to consider:

1. Bacterial Overgrowth

A bacterial bloom is the most common cause. When this happens, in many cases, you will also see white particles floating in your fish tank

But again, bacterial blooms are not necessarily a bad thing. It depends on the factors responsible. For instance:

The ammonia concentration will increase because the water doesn’t have enough nitrifying bacteria. This should change as the cycling process advances.

But some people kill these colonies by removing the filter media, cleaning the filter media with chlorinated water, or adding medicine with toxic elements.

This essentially undoes the cycling process.

The water then becomes cloudy because the bacterial colonies are reforming and fighting against the spiking ammonia levels. 

  • Expect a similar outcome when high temperatures or a poor filtration system (that creates stagnant conditions) depletes the oxygen in the water.

According to James E Alleman and Kurt Preston (Purdue University, School of Civil Engineering), nitrifying bacteria use oxygen to process nitrogenous waste.[1]

An oxygen deficiency makes the water cloudy because it compromises the effects of the cycling process.

2. New Or Dirty Substrate

Some substrates come in packaging with the label ‘Suitable for immediate use.’ But that is not an excuse to use the substrate without washing it.

Residue and dirt on new substrates can add a cloudy texture to the water. The dirtier the substrate, the thicker the gray haze will become.

If you remember cleaning the substrate, the cloudy color probably originates from food and waste that sunk to the bottom. 

Sometimes, these organic compounds only become a problem when you disturb them during a water change.

This usually indicates that the tank is not being cleaned properly.

Cloudy aquarium water in a freshwater tank.

How Do I Get Rid Of The White Cloudy Water?

  • If the tank is new, don’t do anything. Let the tank cycle.[2]

The water will become cloudy as the nitrifying bacteria multiply, but after a few weeks, the organisms will process the nitrogenous waste. 

Then, they will gradually create a clean and stable aquatic environment. Eventually, the milky tone will disappear.

  • Water changes are the fastest solution because they produce immediate results by removing a portion of the cloudy water and diluting whatever remains with new, clean, clear water. 
  • Don’t use chlorinated sources during a water change.

Apply conditioners to tap water. Otherwise, the chlorine and chloramine will make the cloudy tone worse.

You can easily do that with the API STRESS COAT Aquarium Water Conditioner (link to Amazon). That is the product I use each time I’m making a water change.

This conditioner will remove toxins like chlorine and chloramine and make tap water safe for your aquarium fish.

It also creates a protective coat for your fish which becomes extremely useful if they are injured for any reason.

  • Consider adding organic plants. They take carbon dioxide in and produce oxygen.

The plants can prevent oxygen deficiencies if you have a regular day/night cycle (plants take in oxygen in the dark). 

  • Use a strong filter that prevents stagnation.
  • You can replace half of the filter media but not all of it.
  • Wash the filter media using aquarium water.
  • Don’t overfeed the fish. Give them food they can eat in a few minutes twice a day.
  • Don’t crowd the tank. You can get a larger tank or reduce the number of fish.
  • Consider using filters with activated carbon media.

According to a paper in ‘Materials Science and Engineering,’ activated carbon purifies water by consuming organic and inorganic compounds.[3]

Green Cloudy Water In Fish Tanks

Green cloudy water in a freshwater aquarium.

The most common cause of green cloudy water is an overgrowth of algae. But it is worth saying that algae are not always harmful.

A PLoS One paper (Fernando Norambuena, Giovanni M. Turchini, James A. Emery, Yoni Sharon, Karen Hermon, Vanessa Skrzypczyk) found that small amounts of algae in fish feed improve growth.[4]

However, they can also destabilize aquariums by consuming vital nutrients and creating a cover that limits access to light for your plants.

Additionally, the cloudy green texture it creates is unsightly. Algae can thrive in a tank for various reasons, including:

1. Too Much Light

According to a paper in Renewable and Sustainable Energy Reviews, light is essential to algae growth.[5]

The organism uses pigments like chlorophyll to trap light. Therefore, giving an aquarium too much light can increase algae growth.

This is true for both sunlight and artificial lighting sources.

Don’t keep your aquarium next to a window unless you want the water to turn green because of a heavy algae infestation.

2. Plenty Of Nutrients

It could be that your tank has too many nutrients, particularly nitrates and phosphates. Ammonia and nitrites will also contribute to the problem.

A simple inspection will also show that you have decaying matter in the water, probably fish waste, leftovers, and dead organisms.

Leftovers can become a problem if you overfeed your fish. They will disappear into the substrate, where you can’t see them as they decompose.

One expects water changes to prevent these nutrients from running amok. But some water changes make things worse by adding nitrates and phosphates to the tank

When was the last time you tested the water? Tap water simplifies things because you can ask your service provider for a report of the water’s makeup.

But if you get your water from a local spring, pond, or lake, send a sample to a lab and identify the contents of the water.

Many people blame algae blooms on spores that hitched a ride on new plants and ornaments. 

But those spores cannot explode into a full-blown algae infestation unless you create a conducive environment in the aquarium.

Start by testing your aquarium water. For that purpose, I use the API FRESHWATER MASTER TEST KIT (link to Amazon).

I chose this one mainly because it is a liquid test kit, which I found to be the most accurate. These types of kits are also cost-effective because they target multiple substances.

When it comes to nitrogenous compounds, these are the parameters you should aim for:

How Do I Get Rid Of The Green Cloudy Water?

  • Dip plants infested with algae in a bleach solution (5-10 percent). Rinse the plants thoroughly before returning them to the tank. Bleach is toxic to fish.
  • Check your filter. Make sure it is still working. If you recently bought a larger tank, get a stronger filter.
  • Keep the aquarium away from windows. Direct sunlight encourages algae to bloom.
  • If circumstances have compelled you to position the tank next to a window, install thick curtains and keep them closed.
  • Use an automatic feeder to prevent overfeeding.
  • Give the fish food they can eat within two to three minutes. Adult fish should only eat twice a day.
  • Change roughly 15 to 30 percent of the water weekly to prevent nitrogenous waste from accumulating.
  • Add more plants. This gives the algae fewer nutrients to absorb.
  • Add shrimp, snails, plecos, and other species that eat algae.
  • Maintain a regular day/night cycle. Don’t keep the lights on for longer than necessary. Aim for 10-12 hours.
  • Remove decaying matter.
  • Vacuum the substrate at least once a week. You can do that with cheap products like Laifoo Aquarium Vacuum Cleaner (link to Amazon).


White cloudy aquarium water is most likely caused by bacterial overgrowth or your substrate, which may be new or too dirty.

Bacteria can flourish in relatively new containers as part of the cycling process. In this case, there is nothing to worry about.

But if the tank is not new or the substrate is too dirty, the first step will be to clean it and do a partial water change.

Green cloudy water is most likely caused by algae. Fixing the problem focuses on limiting the factors that contribute to algae overgrowth: light, nutrients, etc.


  1. https://www.engineering.iastate.edu/~jea/w3-articles/nitrifier-physiology/nitrifier-behavior.html
  2. https://www.aqueon.com/articles/how-to-fix-cloudy-tank-water-for-a-new-fish-tank
  3. https://iopscience.iop.org/article/10.1088/1757-899X/725/1/012068
  4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4398455/
  5. http://www.jlakes.org/config/hpkx/news_category/2016-03-22/1-s2.0-S1364032115004839-main.pdf