12 Causes of Cloudy Fish Tank Water and What to Do

12 Causes of Cloudy Fish Tank Water and What to Do Advice from Fishkeepup Team

Keeping an aquarium is one of the most fun and rewarding hobbies that you can take up. Not only does it offer endless possibilities for the kinds of environments that you create and the kinds of species that you keep, but it also creates a relaxing feature in your home and introduces an enjoyable routine to your life. Of course, keeping an aquarium is more fun when all goes well. Cloudy water can be frustrating.

What causes cloudy water in your fish tank and what can you do about it? There are numerous potential causes behind cloudy water in your tank. That makes diagnosing the problem the key to finding and executing an effective solution. Taking note of the color and characteristics of the cloud is an important part of diagnosing the problem. Knowing the recent history of your tank helps as well. 

It’s always the case that cloudy water is unsightly. But depending on the cause behind the cloudiness, it could also be the case that you’re dealing with conditions that are hazardous to the health of your fish. In this article, we will examine a dozen of the most common causes for cloudy water in fish tanks and give you helpful suggestions on how to resolve the issue.

Cloudy Water in Your Fish Tank: Causes and Solutions

While it is true that there are numerous potential causes behind the cloudy water in your fish tank, figuring out which one you are dealing with and how to solve the problem doesn’t have to be complicated. Sometimes you can tell by the color of the cloudiness. Other times, you’ll be able to tell based on what you about recent changes in the tank.

#1 – Substrate Residue

If the cloud in your aquarium water is a milky white or grey color, then the problem might be an improperly rinsed substrate. This is especially common with aquarium gravel. Of course, this problem is almost always going to occur immediately after you set up a new tank or introduce changes to your tank’s substrate. It isn’t the sort of problem that just comes out of nowhere after weeks or months.

The best way to deal with this problem is to prevent it in the first place by thoroughly rinsing and cleaning anything that you’re going to introduce into your aquarium. If you fail to get all of the residues prior to setting up your aquarium and end up with cloudy water, you still have options for getting the problem to clear up and they aren’t very tough to do.

In many cases, the cloudiness that is caused by poorly rinsed substrate materials will just clear up on its own over time. Some of it will settle out of the water and some will get caught up by the filtration system. If you need to do more, you can try to use a gravel vacuum to speed up the process. If you do all of this and the problem still doesn’t clear up, it probably isn’t caused by gravel residue.

#2 – Dissolved Solids

You might also hear people call the cause of this problem “dissolved constituents”. Whether you call them solids or constituents, the most likely causes are silicates, phosphates, and heavy metals. If your tank has milky white or grey cloudiness and treating the tank with the solutions for substrate residue doesn’t fix the problem, this is the most likely cause.

The first step in solving a dissolved solids problem is testing to determine what the specific source of the problem is. The majority of the time, you’ll find that your problem can be traced to high pH and that you need to treat your tank’s water with a conditioner. If you are new to aquarium keeping, this is a good excuse to get to know the pros at your local supply store. They can recommend the right products and a quality brand.

#3 – Bacterial Bloom

Both substrate residue and dissolved solids problems are causes of milky clouds in aquarium water but they are both likely to occur very quickly after setting up a new tank or introducing new materials into the aquarium. What if your set-up has been getting along just fine for months and you notice white or grey cloudiness that seems to come out of nowhere?

When this happens, it is almost certainly due to bacterial bloom. Typically, this happens when your aquarium has been up and running long enough to let the bacterial colonies multiply but not long enough for healthy bacteria to establish itself. When your tank is established, healthy bacteria will do the work of filtering and combatting the kinds of undesirable bacteria that lead to cloudiness.

Once again, time is going to be the only remedy that you need in most cases. As the healthy bacteria colonies grow, they will work on clarifying your tank water. Of course, there are some things that you can do to help this process along. Be sure to keep the aquarium clean by removing uneaten food, decaying plants and fish waste. Vacuum the gravel and perform water changes according to your schedule. If you’re still having problems, you might need to use a flocculant.

#4 – Algae Bloom

Bacteria isn’t the only thing that can bloom inside your aquarium and cause cloudy water. If your tank has been up and running for a while and you find that the water is becoming cloudy but with more of a greenish or brownish tint, you’re probably facing an algae bloom. You’ll know for sure if you spot plant-like growth on surfaces inside your aquarium like the glass, rocks, or other decorative items.

If you have living plants in your aquarium then you know how important light and nitrogen are to their well-being. Algae thrive off the same basic elements so it can be particularly complicated to correct an algae bloom in a tank with living plants.

It’s important to understand what led to the imbalance of light and nitrogen in your tank before you take any action to correct the problem. That’s the only way to figure out what you need to do to prevent it from happening again. Once you’ve figured out what is behind the problem, you’ll need to do a complete water change and clean the tank thoroughly.

#5 – Too Much Light

Too Much Light_12 Causes of Cloudy Fish Tank Water

If your aquarium is getting too much light or the wrong kind of light, the symptom that you’ll notice is an algae bloom. Direct sunlight or an overabundance of the wrong kinds of artificial light can cause a nitrogen spike—and as we said, too much light and nitrogen results in algae bloom that you’ll recognize by brownish or greenish cloudiness in your tank’s water.

If you keep your tank in a spot where it receives direct sunlight, you’re going to have to move the tank to someplace where you have more control over the amount and kind of light that it receives. Unfortunately, moving the tank means that you’re probably going to have to empty it. Fortunately, the algae bloom would probably require a total water change anyway – so you’re not really any further behind.

When your tank is already out of direct sunlight, you’ll need to look at your artificial lighting set-up. Whether the bulbs you are using are too powerful for the amount of tank that you have or are simply the wrong kind of bulb putting out the wrong kind of light, getting it right is the only way to solve the problem. There are many Fish Tank lighting quides charts to help you find the ideal set-up for your tank size and type.

#6 – Overfeeding

If you have algae bloom and the problem isn’t with your lighting system then you know that you’ve got an excess of nitrogen in your tank water. The most likely cause of an excess of nitrogen is overfeeding and that should worry you because it not only leads to algae blooms but also threatens to cause harm to many fish species.

Most species are just fine going a day or more without feeding. Some can go even longer than that. What you need to focus on, is paying attention to your fish at feeding time so that you can adjust the amount and frequency. Your goal is to get onto a schedule and routine that results in no leftovers.

Always remove any food that remains uneaten after two minutes. Excess food rots and the process of breaking down leads to the release of ammonia and nitrites into the water in your tank. Healthy bacteria convert some of this to nitrogen. When you get too much nitrogen you get algae blooms.

Overfeeding is a common problem but it is easy to resolve. Keep in mind your fish can get obese (read here) in some point, but that is another story. Once you’ve got a routine figured out that will prevent the problem in the future, you should do a complete water replacement and clean your tank thoroughly to combat the algae bloom. If you catch the problem early enough, you may be able to starve the bloom out and get your water to clear up without a complete water change.

#7 – Nitrates

Excess nitrate levels in your tank’s water will definitely promote algae blooms but you should act quickly to solve this problem because it can also have negative consequences for the health of your fish. It’s easy to diagnose nitrate imbalances with a simple water test.

Use a testing kit to determine what precise cause of the nutrient imbalance is. Whenever you find that nitrates are the problem, a partial water change is the best way to take action. It might take several rounds of water changes, perhaps even an accelerated schedule for changes, before you get the problem cleared up. 

If you’re dealing with a nitrate imbalance and water changes aren’t clearing up the problem, you will need to look into the filter media that are inside of your tank’s filters. You may need to change to nitrate-absorbing and anaerobic denitrifying biofilters. Once you get the problem cleared up, keep a close eye on your tank with regular testing.

#8 – Phosphates

When you conduct a test on your tank’s water, you might find that the problem isn’t nitrate levels but an excess of phosphates. Sometimes excess phosphates come from a source that is easy to fix like excess food that leads to high levels of decaying materials in your tank. Other times, you’ll find that the excess phosphates come from the water source you are using to fill the tank. Be aware of your tap water quality.

If you eliminate the possibility of excess feeding as the source of excess phosphate levels, you will need to look into other options. Unfortunately, none of these other options are cheap or easy. There are some plants and species that you can introduce to try to reduce the phosphate levels. There are also treatments that you can add to your tank water. 

As a last resort, you can look into switching to an RO system to remove the nutrients from your tank’s water. 

#9 – Overstocking

12 Causes of Cloudy Fish Tank Water _Ovrestocking

Once you fall in love with the hobby of keeping an aquarium, it can be tempting to add as many fish to your tank as possible. But this is a mistake and it will quickly lead to water quality problems as the tank is an environment and too many organisms will make it impossible to maintain a balance. Even if a lot of fish means that there is no excess food—it also means a lot of fish waste and that can overwhelm your filters and the tank’s healthy bacteria colonies. Safe fish per gallon rule find in this guide – How many fish can I Fit in One Tank.

Not only will overstocking create water quality problems that will detract from the appearance of your aquarium and reduce your enjoyment of it, but it will also cause stress and competitive behaviors amongst your fish. With the right mix of species of plants and animals, you can maintain an environment with a high number of species, but there is a limit and once you go beyond it, you’ll notice negative consequences.

Of course, when you reach the limit of what your tank can hold but still want to have more fish, you’ve got the perfect excuse to get started on your next big tank

#10 – Driftwood Leaching

If the cloudiness in your fish tank is yellowish or tea-colored, there is only one culprit that you need to consider. Most aquarium supply stores sell African Mopani wood (AMZ) as driftwood for decorating your tank. This particular type of wood can leach tannins into your tank and cause the water to look like a weak cup of tea.

While the tannins are unlikely to cause harm to your fish, it can make the aquarium less attractive to look at. You can do water changes to clear the water up. You should boil or soak the driftwood until you’ve drawn all of the tannins out before reintroducing it into the tank.

#11 – Ammonia Build-Up

When the healthy bacteria in your tank die off or the filters in your aquarium fail for some reason, you can experience excessively high levels of ammonia in your tank water. You will definitely notice signs of distress in your fish if this occurs. You may see cloudiness in the water but it might not be as readily apparent. It’s important that you test your tank water regularly to prevent this as it can be fatal to your fish.

When the healthy bacteria die-off, you will notice more undissolved and undigested solids floating in your tank water. The resulting cloudiness is less of a milky white or grey and less of a brownish or greenish cloud and more appropriately described as just plain dirty looking water.

#12 – Filter Problems

Filter Problem_12 Causes of Cloudy Fish Tank Water

Just as we were discussing with the ammonia build-up, there are times when you might have trouble with your tank’s filter system. When the filter system isn’t working properly or the healthy bacteria have died off, your filter media can become clogged. 

When that happens, the particles in your tank will either pass through the filters or simply fail to be drawn in. Either way, you’ll know you’re having issues from the dirty looking water in your tank.


There is a lot to love about the hobby of aquarium keeping. At the same time, there is plenty that can go wrong and figuring out what the problem is and how to fix it can be very frustrating. If you stick to the advice that we’ve given you in this article, you should be able to identify and diagnose almost any problem that you have with your aquarium. Knowing what to look for is the first step to keeping things on the right track.

It’s important to be very patient while you wait for your solutions to take effect. If you get impatient and through multiple solutions at a single problem, you’re likely to over-correct and create an imbalance in the opposite direction that causes equal or greater problems. Take things slowly and incrementally and give everything a chance to take effect before trying anything else. 


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