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Why You Have A Cloudy Fish Tank (And How To Fix It)


Staff member
Jan 19, 2024
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A cloudy fish tank is something that no aquarist wants to see. Not only is it unsightly, it could also be dangerous!

However, many owners don’t know the first thing about clearing up cloudy aquarium water. In fact, most don’t even know what the cause is in the first place.

But that’s not surprising when you look at the fishkeeping buyer cycle. It’s really not their fault!

Here’s why:

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

It’s a lot.

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

And that puts these owners in a tough spot when it happens to them. They’re on the clock and fighting from behind!

But not on our watch. This guide will go over everything you need to know about the causes of 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 fishkeepers! Most aquarists go to great lengths to keep tanks clean and water conditions stable.

So when that otherwise clear water starts to look murky and discolored, many go into a mild panic!

But first, it’s important to understand that there isn’t a single cause for cloudy water. Many factors can affect water quality. Some issues are benign while others can be a sign of something more serious going on.

Any to figure it out, you’ll need to identify the type of water you’re dealing with.

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

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

White/Gray Cloudy Water​

This is a pretty common type of cloudy water in new fish tanks that were established very recently (sometimes within one day).

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

A betta swimming in a slightly cloudy fish tank

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

In more severe cases, your fish tank can be overtaken with a thick opaque haze! In these instances, the water can look as thick as milk.

So, what causes a white or grey cloudy fish tank?

Likely 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 coated in a fine layer of dust that you need to remove before adding it to your tank. If you don’t do that, the aquarium tank water will cloud up immediately. The dust will float through the water before eventually settling.

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

However, this usually takes several days.

In the meantime, the water return spout will constantly kick up the particles and prevent them from settling!

Author Note: Even if the residue has time to settle, you should always aim to remove it. Movement from your fish will just make the cloudiness return later.

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

For example, hard water is pretty common in the United States. This water usually has dissolved constituents, which could 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 you fill up a glass from the tap? For many homeowners, the water will appear foggy before clearing up a bit. When you’re filling up a massive aquarium, that cloudiness is only intensified!

Use a test kit to analyze your water supply. It will likely be high up on the hardness scale and have a higher pH balance.

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

Before you start worrying, this blossom can actually be very beneficial. When you first set up a tank, the water must go through a nitrogen cycle.

This establishes those all-important bacteria needed to clear waste from the closed environment. These bacteria will turn fish waste into nitrates, making the water a lot less harmful for your fish.

Bacterial blossoms can occur several weeks or even months after setting the tank up. You might also experience cloudiness after a major water change.

Treatment & Prevention​

How you’re going to treat this issue depends on the cause. If residue from your substrate is to blame, take some time to clear things up. You can use a gravel vacuum or store-bought water clarifier. There’s 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 out the substrate thoroughly. Use a sifter or fine-mesh sieve to rinse off any residue left behind.

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

Finally, there’s the issue of bacterial blossoming. There’s actually nothing you have to do to get rid of this issue. Your cloudy fish tank should clear up in about a week!

Author Note: However, if you want to take care of the problem quicker, you can perform a partial water change. This usually stops blossoming in its tracks.

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

Green Cloudy Water​

Green cloudy water is an unsightly issue that can quickly overtake an aquarium. The good news is that the problem isn’t particularly harmful to fish.

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

Likely Causes​

If you’re dealing with a green cloudy fish tank, it’s most likely caused by algae overgrowth!

Aquariums are the perfect environment for algae to flourish. In just the right conditions, this growth can quickly overwhelm the closed habitat.

This isn’t the same type of algae that’s clinging to the glass, it’s phytoplankton. This type of algae is so tiny that you can’t see it with the naked eye.

Author Note: Phytoplankton is a single-cell organism that suspends in water, which is what’s causing the foggy effect.

So why are this algae all of a sudden going crazy in your tank? Well, it could be due to the location of the tank or the condition of the water.

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

The water in the tank may be supporting the algae as well. 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’s something that’s going to take some time to fully get under control.

Treatment & Prevention​

Some algae in your aquarium is fine, but too much of the stuff will make your fish’s home look filthy!

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

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

If the tank is close to a window, consider moving it. You could also use some window coverings or apply a light-blocking material to the side of the tank that’s exposed.

To effectively manage phosphate and nitrate levels, you’ll need to change the way you take care of 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 steps.

Take a look at your filtration media. Chances are, it’s covered in grime! When the filter is unable to catch contaminants, the phosphate and nitrate levels in the tank will rise naturally. Oftentimes, simply replacing the filtration media will do the trick.

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

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

Yellowish-Brown Cloudy Water​

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

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

Likely Causes​

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

One big mistake newer fishkeepers make is overcrowding the tank to create a schooling effect. While many species will naturally group up, there still needs to be some open space.

The more open space there is, the harder it is for waste to sour the water. When you overcrowd fish into a small tank, ammonia and nitrite levels will go through the roof.

And this isn’t just a cosmetic issue.

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

But it might not be all bad.

If you have a well-maintained tank that’s still filled with cloudy brown water, the issue might be caused by staining 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 the trade come from blackwaters that are very rich in tannins. Depending on the species you have, the tea-stained cloudiness could even benefit your fish!

Treatment & Prevention​

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

But realize that there’s no exact science when it comes to this.

Generally, larger fish need more room because they produce a lot more waste than smaller fish. Consider investing in a larger tank or purchasing multiple tanks to prevent overcrowding.

For staining caused by tannins, you’ll need to remove the source. You can get rid of tannins in driftwood by presoaking them. This won’t remove the existing tannins in the water, but it will prevent the problem from getting worse.

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


As you can see, there are a ton of potential causes for a cloudy fish tank. But a lot of them aren’t that bad!

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

Just like anything, it’s always easier to stop a problem from happening than it is to solve it. Be consistent, follow our recommendations above, and your aquarium water will be clear in no time.

If you have any suggestions on ways we can improve this resource we would love to hear from you. Our whole mission is to provide the most helpful information possible, and we’re always open to getting a little extra help!
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