Why you have a cloudy fish tank (and how to fix it)

A cloudy fish tank is something no hobbyist wants to see. Not only is it unsightly, it could also be dangerous!

However, many owners know nothing about cleaning cloudy aquarium water. In fact, most don’t even know what causes it in the first place.

But that’s not surprising when you look at the fish farming buying cycle. It’s really not their fault!

This is why:

When they run out and get a new fish, there are a million things to think about. Care requirements, purchases needed, where to put the tank in your home, you name it!

Its alot.

But nowhere in that process is there a point where someone stops them and explains the possible causes of cloudy water (and what to do about it).

And that puts these homeowners in a tough spot when it happens to them. They are on the clock and they fight from behind!

But not on our watch. This guide will go over everything you need to know about what causes a cloudy fish tank and how you can clear things up.

This way, when the time comes, you’ll be ready!

Why is my fish tank cloudy?

Cloudy aquarium water can be an alarming sight for anglers! Most aquarists go to great lengths to keep tanks clean and water conditions stable.

So when the otherwise clear water begins to look cloudy and discolored, many go into a mild panic!

But first, it’s important to understand that there is no single cause for cloudy water. Many factors can affect water quality. Some problems are benign, while others may be a sign that something more serious is going on.

Anyone to find out will need to identify the type of water they are dealing with.

You can find three different types of cloudy aquarium water. These include white, green or yellowish brown.

Understanding the differences between these cloud problems can help you take action and make changes to prevent cloud problems from occurring in the future.

Cloudy white/grey water

This is a fairly common type of cloudy water in new fish tanks that have been established very recently (sometimes within a day).

The severity of the problem can vary quite a bit. In some cases, the problem is nothing more than a slight mix-up.

Many people don’t even notice the cloudiness at first. Instead, some think it’s just dirty glass!

In more severe cases, your fish tank may be overtaken by a thick opaque mist. In these cases, the water can seem as thick as milk.

So what causes a cloudy white or gray fish tank?

probable causes

1. There are a couple of culprits for this type of cloudy water, and the simplest is residue from your substrate material!

Gravel is usually covered in a fine layer of dust that you need to remove before adding it to your tank. If you don’t, the aquarium tank water will immediately become cloudy. The powder will float through the water before finally settling.

The issue may persist for several days depending on your tank configuration. If you have a filter, the system can remove fine particles.

However, this usually takes several days.

Meanwhile, the water return nozzle will constantly lift the particles and prevent them from settling.

Author’s Note: Even if the residue has time to settle, you should always try to remove it. The movement of your fish will cause the cloudiness to return later.

2. Another probable cause to investigate is the condition of the water you used.

For example, hard water is quite common in the United States. This water usually has dissolved components, which can include everything from metals to minerals. There could also be phosphates and silicates in your water supply.

Have you ever taken a moment to look at the water after filling a glass from the tap? For many homeowners, the water will appear hazy before it clears up a bit. When you fill a huge aquarium, that cloudiness only intensifies!

Use a test kit to test your water supply. It is likely higher on the hardness scale and has a higher pH balance.

3. Finally, the most common cause of cloudy water is a bacterial bloom.

Before you start to worry, this flower can be very beneficial. When you first install a tank, the water must go through a nitrogen cycle.

This establishes the all-important bacteria that are needed to remove waste from the closed environment. These bacteria will convert the fish waste into nitrates, making the water much less harmful to the fish.

Bacterial blooms can occur several weeks or even months after the tank is installed. You may also experience cloudiness after a major water change.

Treatment and prevention

How you are going to treat this problem depends on the cause. If residue from your substrate is the culprit, take some time to clear things up. You can use a gravel vac or a store-bought water clarifier. There is also the option of letting your filtration system do the work for you (if you have a good one).

To avoid this problem in the future, clean the substrate thoroughly. Use a sieve or fine-mesh strainer to rinse off any remaining residue.

To address cloudiness caused by dissolved components, you will need to improve your water supply. You can use a reverse osmosis filter to get rid of any contaminants before adding it to your tank. Alternatively, you can use a water conditioner.

Finally, there is the problem of bacterial bloom. Actually, you don’t have to do anything to fix this problem. Your cloudy fish tank should clear up in about a week!

Author’s Note: However, if you want to fix the problem faster, you can perform a partial water change. This usually stops blooming dry.

You can also prevent future problems by removing debris and keeping the tank clean at all times.

green cloudy water

Green, cloudy water is an unpleasant problem that can quickly overtake an aquarium. The good news is that the problem is not particularly harmful to fish.

The bad news is that it can take some time to get it under control.

probable causes

If it’s a cloudy green fish tank, it’s most likely caused by an overgrowth of algae.

Aquariums are the perfect environment for algae to flourish. Under the right conditions, this growth can quickly overwhelm enclosed habitat.

This is not the same type of algae that sticks to glass, it is phytoplankton. This type of algae is so tiny that it cannot be seen with the naked eye.

Author’s Note: Phytoplankton are single-celled organisms that are suspended in water, which is what causes the fog effect.

So why is this algae suddenly running amok in your tank? Well, it could be due to the location of the tank or the state of the water.

If your aquarium is in direct sunlight, you may be inadvertently feeding too much phytoplankton. More than about 10 hours of light can cause algae to grow and spread at a rapid rate.

The water in the tank may also be supporting algae. Excess nutrients like phosphates and nitrates are food sources that help algae thrive.

There are ways to address the extra nutrients in the water. But it is something that will take some time to fully control.

Treatment and prevention

Some algae in your aquarium is fine, but too much of it will make your fish house look dirty!

The best way to tackle the problem is simply to limit light exposure.

Start by putting your tank lights on a timer. They should be on no more than eight to ten hours a day.

If the tank is near a window, consider moving it. You can also use some window shades or apply a light blocking material on the side of the tank that is exposed.

To effectively manage phosphate and nitrate levels, you will need to change the way you care for the tank. For immediate relief, do a partial water change. This will reduce some of the cloudiness quickly.

However, the phytoplankton will return if you don’t take other action.

Take a look at your filter media. It will most likely be covered in dirt! When the filter is unable to trap contaminants, the phosphate and nitrate levels in the tank will naturally increase. Often, simply replacing the filter media is sufficient.

You can also be more proactive about removing dirt from the tank. Limit feeding to just a few minutes and remove any excess food from the habitat. Get rid of dying plants and any other biological matter that can make the water sour.

Author’s Note: You can also increase the frequency of your water changes. All of these little tasks should help keep phosphates and nitrates to a minimum, which will prevent phytoplankton growth.

cloudy yellow-brown water

The last type of cloudy aquarium water you will encounter is yellow or brown. Again, the severity of this problem can vary widely depending on the cause.

In severe cases, your fish tank could start to look seriously discolored, which would be cause for concern. In other cases, the problem could be something as simple as staining your decor.

probable causes

In the worst case, the water could turn brown or yellow due to overcrowding. No matter how small your fish are, they still produce waste.

A big mistake newer aquarists make is overcrowding the tank to create a school effect. While many species will naturally clump together, there still needs to be some open space.

The more open space there is, the harder it is for debris to sour the water. When fish are crowded into a small tank, ammonia and nitrite levels will skyrocket.

And this is not just an aesthetic problem.

These contaminants will chemically burn the gills of the fish. This will result in a very painful death!

But it may not be all bad.

If you have a tank in good condition that is still full of cloudy brown water, the problem could be due to stains rather than contaminants. Driftwood and decaying leaves produce a substance called tannin.

The tannins are organic and completely safe for your fish. In fact, many freshwater fish in commerce come from black waters that are very rich in tannins. Depending on the species you have, tea-tinted turbidity could even benefit your fish!

Treatment and prevention

To get rid of yellow or brown spots, you need to re-evaluate your tank setup. Do some research to find out how much space each fish needs in your aquarium (start with our library of care guides).

But keep in mind that there is no exact science when it comes to this.

Larger fish generally need more space because they produce much more waste than smaller fish. Consider investing in a larger tank or purchasing multiple tanks to prevent overcrowding.

For stains caused by tannins, you will need to remove the source. You can get rid of the tannins in driftwood by pre-soaking them. This will not remove any existing tannins from the water, but it will prevent the problem from getting worse.

To help clear up a cloudy fish tank, invest in a carbon filter. Charcoal filtration will remove the tannins and make the water crystal clear again.


As you can see, there are plenty of potential causes for a cloudy fish tank. But many of them are not so bad!

The most important thing is to get informed and act when you notice something you don’t like. Even better, engage in good care to prevent this from happening in the first place.

Like everything, it is always easier to prevent a problem from happening than to solve it. Be consistent, follow our recommendations above and your aquarium water will be clean in no time.

If you have any suggestions on how we can improve this resource, we’d love to hear from you. Our entire mission is to provide the most useful information possible, and we’re always open to a little extra help!

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