Cloudy Fish Tank Water Causes and Solutions

Avoid a cloudy fish tank and keep your water crystal clear by practicing good aquarium maintenance habits.
Avoid a cloudy fish tank and keep your water crystal clear by practicing good aquarium maintenance habits.

When the water in your aquarium turns cloudy and your beloved fish disappear into a hazy fog it is easy to get frustrated. Sometimes the reasons why your tank went from fish heaven to fish apocalypse may be as clouded as the tank water itself.

Rest assured, tank water does not change so drastically without reason. This means you probably did something wrong. But it also means that, once you understand your mistake, you can change your tank management practices and greatly reduce the chances of it happening again.

In this article you will read about five common reasons fish tanks turn cloudy. One or several of these causes may be at the root of what has happened to your tank. In many cases these problems are easy to solve, and if you act soon enough your fish should pull through without any trouble.

Throughout this article you will see two pieces of advice repeated no matter the cause of your cloudy water. First, you need to perform a partial water change. This will dilute the dissolved chemicals and waste in the water and make it a safer environment for your fish. While it may be tempting to remove all or most of the water in an attempt to clear things up, this can potentially make things worse. A 25% water change is a good starting point.

Second, use a gravel vacuum to clean the gravel and remove as much of the fish waste and decaying matter as possible. When this junk is left to accumulate on the substrate it can eventually cause tank-wide issues. With most of the good vacuum systems you can change the water and clean the gravel at the same time.

Regular water changes and gravel vacuuming are two of the most important duties of your aquarium care routine. However, there are often other factors that can make your tank water turn cloudy, and unless you eliminate the problem it is likely to occur again.

As you will see, some of the issues that cause an aquarium to turn murky are symptoms of greater problems.

Bacterial Bloom

We'll start with the most common issue that may have turned your tank cloudy: Bacterial bloom. Decaying food and fish waste provides a rich source of nutrients for bacterial growth. A bacterial bloom usually appears as a milky haze.

Bacteria in your tank isn't a bad thing. In fact, it is needed to break down waste and maintain healthy water conditions. The balance between microorganisms and the fish in your tank is what keeps harmful chemicals under control.

But when something tips the scales and the bacterial colonies in your tank can't get the job done, that's when water quality suffers. Waste builds up, and the bacterial population suddenly explodes with the abundance of food.

Most commonly, a bacterial bloom occurs when a new tank hasn't been cycled properly, or when a massive water change and aquarium cleaning results in a mini-cycle as the bacterial colonies in your tank go through a stage of re-population. This is a big reason why you always perform partial water changes when performing regular tank maintenance: You don't want to remove too many of the microorganisms that are keeping your tank healthy.

So what can you do? If you have just performed a big water change or if your aquarium is new it will simply take a little time for your tank to come in balance. The cloudiness should clear up on its own in a few days.

However, if you believe the bloom is due to factors such as excess waste matter accumulating on the substrate or even a dead and decaying fish, you need to take action. Perform a partial water change and get out your gravel vac.

Regular partial water changes help to dilute harmful waste chemicals and keep your water healthy. But changing out too much water can have a negative impact on healthy microorganisms in your tank.
Regular partial water changes help to dilute harmful waste chemicals and keep your water healthy. But changing out too much water can have a negative impact on healthy microorganisms in your tank.


Feeding your fish too much and too often in itself can cause water cloudiness. This may be due to food particulates dissolved in the water, a bacterial bloom or both.

The instructions on many fish-food containers tell you to feed multiple times per day. That’s fine, but if you are going to do that you really need to get a handle on how much your fish are eating, and how much is drifting to the bottom of the tank. Uneaten food decays, and enough of it can cloud the water.

This can happen in a large tank, but it is also the kind of issue that often happens to well-meaning Betta owners. One little Betta fish does not eat a whole lot of food, so giving him a pinch of fish flakes a few times a day will soon pollute his little tank.

To remedy this problem perform a partial water change and vacuum the gravel in order to clean up the tank and make it safer for your fish. Then, you need to reevaluate how you are feeding.

I prefer to feed once per day, and allow them one day of fasting per week. This is healthier for your fish than overfeeding, and will help keep your tank cleaner. Remember, some of what goes into a fish is eventually going to come back out. Feeding too much food not only means more decaying, uneaten food in the tank, but also more fish waste.


Overstocked fish tanks have multiple issues. Aside from behavior clashes and stress for the fish themselves, the tank conditions in an overstocked tank can often deteriorate to a toxic environment which can result in cloudy tank water.

To understand the problem, think of your tank as a little ecosystem, something like a tiny lake. Your tank has microorganisms that help to break down waste, and the filter systems serves in the role of the streams and springs that cleanse a lake and aerate the water.

Just as a lake can only support so much life, the same is true of your tank. A lake is more resilient because it has the advantage of a constant supply of fresh, clean water. But, because your tank is a closed ecosystem with no fresh water coming in, once things are out of whack it is unable to regain that balance without your intervention.

Some fish keepers make up for overstocking by over-filtering and keeping a heavily planted tank. Both of those things help, but ultimately the solution is to keep your stock down to a level that the tank can maintain.

Perform a water change and vacuum the gravel to get things back somewhere close to normal, but then find a way to re-home some of those fish.

Algae growth is typical in aquariums, but a sudden algae bloom can turn your water a murky green.
Algae growth is typical in aquariums, but a sudden algae bloom can turn your water a murky green.

Algae Bloom

If you have run an aquarium for any amount of time you are likely well aware of algae. These plant-like organisms grow on your decorations and the sides of your tank, and left unchecked can turn your tank into a green mess. Algae blooms often appear as a greenish hue.

Algae thrive on the same things that plants love: Food in the form of nitrogen, and sunlight. If you are having a problem with algae blooms in your tank the cause is probably related to one of those two factors.

A sudden spike in nitrogen in the form of excessive feeding and/or fish waste (see above) may also cause a sudden algae bloom. Remember that algae aren’t just a nuisance, they are living organisms in your tank and therefore impact the ecosystem just like fish or plants. If you do nothing a sudden increase in algae can upset the balance of your tank, and this is bad for your fish.

Another reason you may experience frequent algae blooms has to do with your tank placement. Your tank should not be hit by direct sunlight during the course of the day. If it is, this gives algae some added energy to get growing. Remember, like a plant, if it has food and sunlight it can thrive.

The solution to an algae bloom is to perform a water change and then remedy whatever situation caused the bloom. This means either get your feeding practices under control, or move your tank to a location where sunlight won’t help algae run amok.

Driftwood Leaching Tannins

Here’s a reason a tank may turn cloudy which really isn’t a care mistake on your part, but may instead be due to a lack of knowledge of your tank decorations. Using natural driftwood in your tank is a great idea. It looks fantastic, and some fish will benefit from nibbling or rasping on it. Driftwood can also drop the pH of the tank water slightly, which is useful if your water source is a bit hard like mine.

But if you’ve just added a new piece of driftwood to your tank you may notice your water has turned a yellowish tea-like color. This is because driftwood, especially commercially available types like African Mopani, leach tannins into the tank water.

This isn’t likely to harm your fish, so you can wipe some sweat off your brow. But if you don’t like the hue your tank has taken on because of the tannins, but you do like the driftwood, there are a few things you can do.

Common advice is to boil the driftwood before dropping it into your tank. However, sometimes the wood you have chosen is too large. In these cases, I’ve used a large storage container to soak a new piece of driftwood for several days before placing it in the tank. As with everything else that touches your tank, be sure to use a container that does not have a buildup of detergents or other chemicals.

If you have carbon in your filter it can help a little, and regular water changes (not more than weekly) will gradually clear up your water. Driftwood stops leaching tannins over time, so eventually this will cease to be an issue.

Avoid a Cloudy Tank

As you’ve surely deduced by now, there are a handful of basic things you can do to greatly increase the chances that you never wake up to an aquarium that has suddenly turned cloudy:

  • Keep up with tank maintenance. This means a regular schedule of water changes and gravel vacuuming.
  • Do not overfeed your fish. Excess food means more waste.
  • Do not overstock your tank. Too many fish means your tank can’t keep up with the processes required to keep the water healthy.
  • Do not place your tank in direct sunlight. Just like plants, algae thrive in sunlight.

Of course we all falter from time to time. Life gets busy and you neglect to perform simple aquarium maintenance chores, or a personal or business emergency takes your time away from your fish keeping hobby. If this happens, all is not lost. Once things settle down you can usually get your tank back on track.

How often do you experience a cloudy aquarium?

  • Constantly it seems!
  • Every now and then.
  • Only a few times.
  • This is the first time!
  • Never!
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Comments 2 comments

Pollyanna Jones profile image

Pollyanna Jones 14 months ago from United Kingdom

This is a really helpful guide. It's all ups and downs and it has gone a little bit cloudy at times! We bought our first aquarium last month and are trying to get the levels right before we put fish in.

Kevin Goodwin 14 months ago

As long as you take proper care of your tank and test the water and not overfeed or crowd your tank you should be good.

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    EricDockett profile image

    Eric Dockett (EricDockett)293 Followers
    62 Articles

    Eric is an aquarium enthusiast with over two decades of experience caring for a wide array of tropical fish.

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