Why Does Aquarium Water Get Cloudy?

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What Causes Cloudy Fish Tank Water and How Do You Clear It Up ...

As an aquarium owner you look at your most often and wonder – Why Does Aquarium Water Get Cloudy?

Cloudy water can be a baffling phenomenon for many aquarium owners. Unfortunately, there is no single answer as to why your aquarium water is cloudy because there is no single cause.

However, based on the color and circumstances under which cloudy water appears, it usually can be pinpointed to a couple of basic causes.

When You See White cloudy water:

Sometimes, you can barely see it, a slight grayish haze… showing white cloudy waterOther times, it looks more like milk than water.

Seriously, there are fish in there… Somewhere.

Here are the reasons why your aquarium looks like it has been blanketed in white fog…

1. Unwashed substrate

Did you forget to wash your substrate before adding it to your aquarium?

If the water turns cloudy within two hours of setting up your aquarium, then this is probably why.

You see, gravel and sand substrates contain really fine specks – created when it rubs together…

Now, these dust-like pieces are so small that you won’t even notice them at first. However, when added to water, they separate from the larger pieces of substrate and float around your aquarium.

The result?

A very visible and ugly haze.

I know it’s too late now, but this is why you should wash sand and gravel substrates before adding them to your aquarium – it removes the fine dust.

How to get rid of it!

Gravel and sand residue is perhaps the easiest cause of cloudy water to fix. A water change will help remove a portion of the dust that is floating around your tank

However, the best solution is to just wait. If your filter uses a fine mechanical media, such as filter floss, then it will eventually trap most of the dust that is floating. You could also use a good water clarifier to speed up the process.

Now, it’s likely that a small amount of the dust will settle on the floor of your tank. This dust will again kick back up into the water if it’s disturbed. Not to worry! This can easily be removed with a quick gravel vac – check out our gravel vacuum review to find the perfect one for your tank.

2. Bacterial bloom

If your water looks like a thick white fog, then you might be dealing with a bacterial bloom. Yep, that hazy cloud could actually be millions of tiny bacteria, swirling around your tank.

On its own, a single bacteria is invisible. However, as they grow in number, the group looks like a foggy discoloration in your aquarium.

Gross, huh?

What you are looking at is commonly called a bacterial bloom.

Bacterial bloom is a common sight when cycling your aquarium. In some cases, it will be a mild haze, while in others, your aquarium can look like it’s filled with milk – it all depends on how many bacteria there are.

How to get rid of it!

You do nothing. In a cycling tank, this bacterial bloom will disappear on its own. A week later, your cloudy water will be nothing more than a distant memory.

If you are not cycling your tank, then a bacteria bloom could be a big warning sign. You see, this bacteria commonly appears when decaying plants, rotting fish food or too much poop builds up in your tank.

If that’s the case, the first thing you want to do is grab your trusty aquarium test kit.

Got it? Good. Now, you need to check your ammonia and nitrite levels to make sure they are both at zero parts per million (ppm).

If they have risen, perform an immediate water change so that no harm comes to your fish. Now, you have to figure out what the cause was.

This cloudy looking bacteria often appears when there is too much waste breaking down. The most common cause of this is overfeeding your fish. Not only will your fish poop more, but there will be uneaten fish food rotting at the bottom of the tank – fix this by cutting back feedings and removing all the uneaten food and decaying waste from your tank.

Another cause could be that you accidentally killed off all the good bacteria in your tank. If you rinsed your aquarium filter media in tap water, the chlorine will kill the good bacteria, and you will have to cycle your aquarium all over again, with your fish still inside.

When You See Green cloudy water

Murky green cloudy water

Green water is a no-brainer. It’s due to algae growth. Getting rid of it is the hard part, but if you know the cause, it’s easier to cure. Here are the primary causes of green water:

Out of all the different colored clouds that will visit your tank, green cloudy water makes beginners panic the most. It just doesn’t look right, like pea soup.

In severe cases, your water will be so murky that you can barely see your fish.

So, what is this weird green fog that covers your aquarium?

Well, it turns out that it’s a particular type of algae…


Phytoplankton is small. Really small. In fact, you can’t see it with your naked eye.

But as it grows in number, it will become very noticeable, turning your water a cloudy green color.

But don’t worry! While it may not be pleasant to look at, this algae won’t harm your fish.

Too Much Light

The most obvious cause and the easiest one to cure is too much light. Placing the aquarium in direct sunlight or leaving the lights on too long will result in algae growth. Reduce the amount of time the lights are on, and move the aquarium to a location out of direct sunlight.

Excess Nutrients

Nutrients such as phosphates and nitrates also support the algae growth and must also be reduced to successfully battle algae. A water change will give some immediate relief but probably won’t resolve the problem completely. It’s important to deal with phosphates and nitrates at their source to rid yourself of them.


Phosphates come from two sources—decaying matter such as fish food, and from the water source itself. Testing your tap water for phosphates will let you know if you have a problem with your water source. If your water naturally has a high level of phosphate, you will need to use RO water or a phosphate remover to treat the water. Reducing the amount of food you give your fish, and changing to a brand that is lower in.


Nitrates naturally rise in the aquarium over time as a byproduct of fish wastes. The only way to remove them is to perform a water change. Make sure your filter is kept clean and is adequate for your tank size. Also, make sure you have not overstocked your aquarium, or you will constantly battle rising nitrate levels.

A vast majority of cases of cloudy water can be resolved by weekly 10 to 15 percent water changes, keeping the gravel very clean, and using good quality food.

How to get rid of it!

A UV sterilizer or fine filter media, like a polishing pad, will quickly put an end to this green cloudy mess for good – your water will be crystal clear in no time.

While this will fix the problem – the algae – it doesn’t stop the cause.

Unfortunately, there are a lot of different causes…

  • Dirty filter
  • Not performing water changes
  • Too much light
  • Carbon dioxide (CO2) deficiency
  • Imbalanced fertilizers

Most of the time, one of these is responsible for your murky water. Sort these out to prevent the green cloud from coming back!

When You See Yellow or brown cloudy water

Yellow cloudy aquarium water

While it might start out as a yellow tinge, if you don’t stop it, your water will darken in color until it looks like a refreshing sweet tea.

The likely culprit?


You see, wood, leaves and seed pods release a brown dye when soaked in water. This brown dye is called tannin.

As the tannins slowly leach into your aquarium, it will turn your water a yellowy brown color.

FishLab Fact: Some aquarists want their aquarium to turn a brown color, adding untreated driftwood and leaves to darken the water. These tanks are known as blackwater aquariums, and while they may not appeal to everyone, they are beautiful when designed correctly.

How to get rid of it!

Exactly how many tannins are released entirely depends on the type of driftwood. Some woods, like manzanita, barely color your aquarium while others, like Malaysian driftwood, paint your aquarium a cloudy shade of brown.

Fortunately, most tannins can be removed by pre-soaking the wood before placing it in your aquarium. Boiling speeds up the process.

If it’s too late, and you already placed the wood in your aquarium, remove it and give it a good soak. Any tannins that you remove now won’t dye your aquarium water when you return the driftwood to your tank.

But what about the water inside your aquarium… The stuff that has already been dyed a brown or yellow color?

Well, using a carbon filter will remove any color from the tank, leaving you with crystal clear water.

When You See White or Grayish Water

Gravel Residue

If the water is cloudy immediately or within an hour or two of filling the tank, it’s probably due to insufficiently washed gravel. Drain the tank and rinse the gravel until the water runs clear. That should resolve the problem.

Dissolved Constituents

If washing the gravel doesn’t solve the problem, the next most likely cause of cloudy water in a newly filled tank is a high level of dissolved constituents, such as phosphates, silicates, or heavy metals. If you test the water, you’ll likely find that the pH is high (alkaline). In these cases, treating the water with conditioners will often resolve the problem. Another option, which has many benefits beyond resolving cloudy water, is to use RO (Reverse Osmosis) water. Your local fish shop may sell it or sell units capable of making RO water.

Bacterial Blossom

Often, cloudy water doesn’t appear the instant an aquarium is set up. Instead, it appears days, weeks, or even months later. The cause is usually due to bacterial bloom. As the new aquarium goes through the initial break-in cycle, it is not unusual for the water to become cloudy or at least a little hazy. It will take several weeks to several months to establish bacterial colonies that can clear waste from the water. Over time, that cloudiness will resolve itself. Decaying plants or excess food that remains uneaten can also cause the milky water seen in bacterial bloom.

Regardless of the cause, don’t panic over bacterial blooms. Keeping the aquarium very clean by removing debris such as decaying plants and uneaten food, vacuuming the gravel regularly, and performing partial water changes will quickly resolve most cases of bacterial bloom. Cut back feeding to every second or third day, which will reduce excess food decay. If there are particles of debris in the water that you are unable to remove via water changes and vacuuming, a flocculant may be used to clear them away. Flocculates cause particles of debris to clump together so they can easily be removed by the filter (be sure to clean your filter so it’s working at peak efficiency). Flocculates are generally marketed as water clarifiers and may be found at your fish shop.


The Best Solutions for Cloudy Aquarium Water

A natural reaction is to “do something”.  There is obviously something “wrong” with the tank, requiring action on our part. However, prevention/avoidance is far better than attempting to mitigate cloudy water once it starts. When you begin to see cloudy aquarium water, it’s best to do nothing and let it run its course, but continue reading for some additional insight.

Should I do nothing and let nature take its course?

  • Yes! Without question, doing nothing is the best approach for a new fish tank. Cleaning the filter does nothing except disrupt the few beneficial bacteria that have had a chance to get established. These “good guys” will eventually outcompete the cloudy water bacteria for food, starving them out and breaking down their carcasses.
  • Water changes clear the water temporarily, but in a day or two the cloudiness reappears, often even worse than before. That’s because the new water provided a fresh supply of nutrients, causing the cloudy water bacteria to populate even more.
  • Left alone, the cloudy water bacteria will eventually consume all the nutrients in the water and die out. This is cycling!

Should I add live plants or other beneficial bacteria?

  • Yes! Live plants have “good” bacteria and other microbes on them, which help initiate the biological balance in the aquarium.
  • Live plants compete for nutrients and help starve out microbes that cause cloudy water.  In addition, they produce oxygen during the day, which aids in the breakdown of fish waste, uneaten food, and the cloudy water bacteria as they begin to die off.  This third benefit helps clear the water.
  • They also consume ammonia generated by fish and uneaten food, which tends to build up in newly set up aquariums until the nitrifying bacteria become established.

close up of green aquarium plant

Should I add or change filtration? 

  • No! The big thing in terms of the filter when dealing with “New Tank Syndrome” cloudy water is don’t mess with it.
  • Cleaning a brand new filter or replacing the pad does nothing good, and potentially eliminates the good bacteria that are trying to get established. If the filter pad or media are in need of cleaning sooner than the first 30 days, you are overfeeding, overstocking, or both.

Should I change the water more often?

  • No! Regular partial water changes (at least 25% monthly) are the #1 thing aquarists should do to be successful, EXCEPT during new tank syndrome. As mentioned above, the water clears temporarily (24 hours at best), but the cloudiness comes back with a vengeance because you just gave it a boost of nutrients with the incoming water.

clear looking fish tank

How to Prevent Cloudy Fish Tank Water

1. Do not overfeed your fish

Beginning aquarists often fear their fish will starve to death, so they feed heavily and often.  Unfortunately, there are few, if any, nitrifying bacteria present to break down the resulting waste or uneaten food, which the cloudy water bacteria take advantage of and continue to multiply. Even worse, harmful ammonia and nitrite levels may begin to rise. Fish in nature don’t always eat every day, and some predatory fish may only eat once or twice a week. No fish ever starved to death in three days.

2. Don’t put too many fish in your fish tank.

More fish mean more waste and more food for the microbes causing the cloudy water. Too many fish in your fish tank may also cause a rise in harmful ammonia and nitrites.

3. Add activated carbon media to the filter, whether loose or carbon pads.

Adding activated carbon media or activated carbon pads to the filter will help clear the water and adsorb nutrients that feed the bacteria bloom.

4. Seed the aquarium.

If you have access to another healthy, well-established fish tank, adding a few handfuls of gravel from that aquarium will seed the beneficial bacteria and speed up the clearing process. Aquatic stores sometimes keep filter cartridges, bio-sponges and wheels floating in stocked aquariums to seed them with bacteria. Then, they could send these items home with new setups to help get the biological balance going. This is the same effect as adding gravel from an established tank.

5. Test your aquarium water.

Have aquarium water tested for ammonia and nitrite as soon as the water begins to get cloudy. In most situations the levels will be zero, meaning there is no cause for concern.


As you see, cloudy aquarium water comes in many different colors.

And while it might look ugly, you won’t have to put up with it forever. Whether it’s milky white or pea green, it’s easy enough to stop in its tracks.

We understand that seeing cloudy water in a new aquarium, can be alarming. But the best advice is to be patient and wait it out. Don’t add any more fish, feed sparingly once every other day, and just leave the filter alone for the time being.


See Related Post:

Best Aquarium Water Clarifier

Best Aquarium Glass Cleaner And Algae Scrapper

Best Aquarium Test Kits and Strips

Best Aquarium Aquascaping Tools Kits

Best Aquarium Ammonia And PH Alert


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