Cloudy Fish Tank Water: Causes & Solutions

Keeping fish has been a hobby for thousands of years.  The process and techniques for keeping fish though has changed a lot over all that time.  Keeping fish, especially in aquariums, has jumped in popularity in the last one hundred years or so and it seems so many people keep fish in their home or office nowadays.  Generally speaking, most fish and other aquatic creatures are fairly easy to take care of as they only need the basics in feeding and cleaning.  Many tanks only require a minimal amount of care to keep everything healthy and thriving.  However, there is one issue that many new fish hobbyists are not aware of and should be.  The water becomes a grayish or milky white color, as it kind of looks like someone poured milk into your tank.  Or the water seems yellowish, or even green.  This is fact that after having fish for a certain amount of time, the fish water tank is cloudy.  This cloudy aquarium water is a very common problem and is one of the first signs that the tank needs to be tended to. 

Is Cloudy Fish Tank Normal?

You will know when you see it.  The cloudy aquarium water makes the view inside the tank unbearable.  The fish and other aquatic creatures inside the aquarium can barely be seen and it makes the entire tank look dirty and uncared for.  Don’t worry though.  This is a very common issue with fish hobbyists and is actually fairly easy to fix.  In fact, some hobbyists look at cloudy fish tank water as a rite of passage to being a true aquarist. 

If you do not fix the issue, then this could spell very bad news for your aquatic friends.  There are many reasons why your tank might have these issues and it is vitally important for the health of all the living things that live in your aquarium.  This guide will educate you on what different variables cause your tank to discolor and help your tank avoid cloudy aquarium water.

Cloudy Fish Tank
Cloudy Fish Tank

Causes of Cloudy Fish Tank

There are many reasons why fish tanks can get cloudy. Causes of a cloudy fish tank includes, bacterial bloom, overfeeding, too many fish, inadequate filtration, dead fish or decaying matter.

Bacterial Bloom in an Aquarium

Bacterial bloom in aquariums is going to be the most common thing that you run into when dealing with an aquarium.  Bacterial bloom can happen in both a new tank setup or in an established tank, but 90% of the time it is going to happen when you are initially setting up a new tank.  Also known as a bacteria blossom, this is when certain unwanted bacteria feed on the various nutrients.  Bacteria blooms often happen when you are cycling an aquarium or setting up a new one.  Bacterial blooms are expected when it comes to new tanks, but if you are getting bacterial bloom in an established tank, then there is something that is causing the heterotrophic bacteria to multiply.

Don’t know what I’m talking about?  Well, in order to fully understand how these bacterial bloom in aquariums occur, you must first understand the difference between heterotrophic and autotrophic bacteria.  Heterotrophic bacteria feed upon all kinds of organic matter: dead fish, old food, dead plants, and fish poop.  Anything organic that resides in your tank will be gobbled up by these heterotrophic bacteria.  The reason these bacteria are bad is because they break down all of that organic matter into ammonia.  These kinds of bacteria are the ones that cause the clouding of the tank.  However, autotrophic bacteria feed on inorganic matter like ammonia, nitrite, and nitrate.  The autotrophic bacteria are way more beneficial for your tank anyway.  The autotrophic bacteria help regulate the water levels and keep the fish tank healthy.  So if you are seeing that the fish tank water is cloudy and white or grayish, then the balance between the heterotrophic bacteria and the autotrophic bacteria becomes disturbed because the heterotrophic bacteria population grows too large.  This is where the term bacterial bloom comes from because there is a literal explosion of heterotrophic bacteria inside the tank.

As mentioned before, this happens 90% of new tank cases, so there is no need to get excited.  The reason behind that is because the new tap water that you are adding to the tank is very rich in nutrients.  The nutrients are then eaten up by the heterotrophic bacteria and causes them to rapidly reproduce.  After all, they are feeding on the plentiful amounts of delicious organic matter.  This process happens very quickly, as the number of these bacteria generally double every 15-20 minutes.  In fact, when the heterotrophic bacteria population reaches a certain point, they become visible to the naked eye.  As they continue to reproduce from feeding off the organic nutrients in the tank, the heterotrophic bacteria can be seen with the naked eye as a cloudy, milky fog.  The result of that population growth is the cloudy aquarium water in your tank and this precisely what you need to get under control if you are to get rid of that cloudy aquarium water. 

The solution to this problem is actually quite simple: do nothing. That’s right, not a thing.  No additives.  No extra chemicals.  Please do not attempt to change the water because changing the water will only make things worse. This is because you are adding new nutrient-rich water into the tank for the bacteria to feed on.  If you add this nutrient-rich water again, you would be restarting the cycle again and would have to wait longer.  So, don’t do anything to the water.  Remember that nature has a funny way of working itself out.  In a new tank, the cycle of the water in the tank will correct itself.  The autotrophic bacteria will eventually grow over time and continue feeding on the inorganic matter, eventually starving out the heterotrophic bacteria.  This process will allow the water to rid itself of the fogginess on its own.  And it will happen fairly quickly.  One day, you will wake up and the cloudy aquarium water is gone. 

On the other hand, if you have a well-established tank and been keeping aquatic creatures in there for some time and you get bacterial bloom, then something might not be right about the ecosystem of your aquarium.  Somehow, some additional organic nutrient has found its way into your tank and is causing the bacterial bloom to occur.  To correct this, you will need to do a bit of investigating with one or more of the following suspects in this cloudy mess.  The most likely culprit of this would be overfeeding the fish. 


Overfeeding is another common issue with people with fish.  Overfeeding any animal is never a good idea.  Some animals can just shake it off or even stop themselves from eating anymore. Other animals are stuck in survival mode and will eat almost whatever is put in front of them.  Unfortunately, sometimes the wrong amount of food is put into the tank.  This is bad for two major reasons.  One is that your fish will be overfed, which can cause health problems.  The other reason is that this overfeeding might be the organic nutrients heterotrophic bacteria are looking for.  Maybe some food fell behind a rock or in another unpopular area of the tank?  This is why it is important to consistently monitor how much food Is going into the tank.

Too Many Fish

Another cause of bacterial bloom is overcrowding your tank with too many items.  Too many fish, for example, adds more waste and more food to the tank for the bacteria to feast on, which develops into cloudy fish tank water.  Too many fish can also cause a rise in harmful ammonia and nitrates.  Make sure not to add too many fish into your tank at once.  It is important to allow the tank to get used to the creatures inside the tank before adding new ones.  Generally speaking, you do not want to add more than 50% of your fish population to your tank and spread out the distribution of those fish over a 4-6-week timeframe so the tank has time to balance out.   If your tank is just starting out, then you may want to consider introduction periods.  This is the process where you add several fish together, wait for a few weeks for things to settle, and then add several more.  The main reason why this happens is due to poor planning from the owner of the tank.  So it is best avoid adding too many fish at once.

Inadequate Filtration

One of the more overlooked, but obvious answers to bacterial bloom in aquariums is an inadequate filtration system.  When you buy cheaper power filters for your tank, they use coarse foam, which finer particles can pass through easily.  However, if you can afford the higher end brans like the Bio master Thermo, they will have multiple foam densities that you can choose from.  You will probably need the finer foam if you have a heavy populated tank.  If you are using a power filter, you might want to consider adding an additional layer of filtration to improve the mechanical filtration.  Keeping an eye on your filtration unit will be very important in keeping your water clear.

Dead Fish or Decaying Matter

Another major factor for the bacterial bloom is not keeping up with cleaning the tank.  Obviously, if you do not keep a clean tank, then it will affect the health and well-being of all of the aquatic creatures inside.  Quite often, there will be a dead fish or some kind of matter that is hard to see debris that has fallen out of sight behind some plants or rocks.  If you do not clean your tank out, then this dead matter can have serious repercussions on your fish and tank.  Cleaning the tank is something that should be done on a regular basis anyway, but if you are seeing the cloudy fish tank water, then this may be the issue that needs attention.

How to fix cloudy fish tank caused by bacterial bloom?

The somewhat complicated aspect about having a fish tank is that if a problem comes up, then you need to be an investigator and try to see what is causing the problem.  Once you have localized the issue, you will need to take the correct steps to fix the problem.  If the bacterial bloom comes from new tank water, do nothing.  If the issue is overfeeding and you are seeing too much excess food in the tank, then you will need to clean out the tank and lower how much food you are giving your fish.  If the issue is a mechanical problem, like filtration, then you will need to read the manuals and make sure that they are not broken or installed incorrectly.  Or if the problem is dead matter within the tank, then you will absolutely need to clean the tank out.  All of these problems are fairly easily fixed, but the results might take some time to manifest.  Depending on the issue, the solution might take a few days to get rid of its cloudy aquarium water.  Once the problem is handled, make sure to note the issue and keep an eye on any future changes the water brings.  However, in most cases, it should not take long for the tank to correct itself once all the problems have been addressed. 

Fix Cloudy Fish Tank
Fix Cloudy Fish Tank

Cloudy Fish Tank Caused by Water Agitation

Surface agitation is a concept that is often sent to the back burners with most aquarists and is often forgotten about when people are starting up a new tank.  Surface agitation occurs when movement of water on the surface of the tank.  This is an important piece to know because this actually oxygenates your water.  Surface agitation promotes a gas exchange where the agitation increases the rate at which oxygen dissolves in water. It also increases the rate at which carbon dioxide is released from the water.  The amount of oxygen in the aquarium determines how many fish can be in your tank.

Your fish still need oxygen to survive, just like us. Low levels of oxygen in your tank will stress your fish out.  This leaves them open to diseases, fungus, bacteria, and other issues.  By disturbing the surface of the water in the tank, it will allow for the carbon dioxide to escape the water and oxygen to settle into the aquarium.  Therefore, this goes back to the concept of proper filtration and movement of the water in the tank to break up the surface tension.  You cannot just throw water into a tank and call it a day.  Cloudy water will dissipate from new water over time after you complete a water change. 

Once again, don’t do a thing, but make sure you are making sure the filtration system and all other mechanisms working in your tank are working properly.  The cloudy water will correct itself and should only take a short time before the issue is fixed. 

Cloudy Fish Tank Caused by Aquarium Gravel

There are really two major things when it comes to using gravel in your aquarium.  One major cause of cloudy water is putting in new gravel for the first time.  The gravel needs a bit of time to settle, so if you are putting new gravel into your tank or you are setting up your tank for the first time, don’t worry.  The cloudy water will go away once the particles from the gravel settles on the bottom of the tank. 

The other reason gravel is a cause of cloudy fish tank water is because of the gravel getting grimy and gross on its own over time.  You must think of a fish tank like a room in your house.  Sometimes, it just needs a deep cleaning.  Debris, crumbs, and gunk get everywhere and it is kind of starting to stink.  Part of that might be due to the gravel needing a cleaning.  Since gravel has spaces in between, there is a high chance that pieces of food, dead cells, or whatever get trapped in between and start to affect the environment.  Remove the gravel from the tank and make sure to rinse it out really well before putting it back in.  This will clean off the excess junk that has attached itself to the gravel.  Rinsing out your substrate and gravel is one of the best ways to prevent the water from clouding up.

Cloudy Fish Tank Caused by Green Water Algae

Have you looked in your tank and saw that the water was green?  It kind of looks like someone poured Mountain Dew into the tank.  While this green water looks gross, it is not inherently harmful to the aquatic creatures living inside. The green water is usually cause from one of several factors.  The major suspects of the green water effect are usually light, nitrates, and phosphates.

If you plan on starting a new aquarium, it really helps to plan out your tank in advance, so that you understand what exactly the tank and the fish are going to need.  When it comes to light, you want to make sure that your tank is lit well, but not so overly lit up that it causes a problem for the fish.  Using light that is too intense or leaving the light on too long can create an algae bloom, which in turn turns the water a greenish color.  Remember that you are recreating a natural habitat for these fish.  In a natural setting, the sun will go down and darkness will occur.  Therefore, it is important to keep the same schedule for your own fish.  The tank itself only needs about eight hours of light per day.  This will help you avoid any algae build up or green water.

A second suspect is the nitrates within the tank.  Nitrates will build up in the water column over time.  To avoid this, you really need to be consistently changing the water on a regular basis.  This will prevent too much nitrate build up, which the algae thrive on.  By changing the water, you will be able to avoid any greenish water.  To get rid of the green water, it is suggested to change about 50% of the water every three days and make sure that the substrate is clean.

The last major suspect in green water is the phosphates.  This is another nutrient that algae really love to feast on.  In fact, phosphates tend to beef up the algae growth and act as a sort of steroid for algae.  This goes well with the description earlier about getting rid of dead plant materials, dead fish matter, uneaten food, or fish waste.  In order to be sure that phosphate levels do not grow out of hand, it is paramount to not overfeed your fish, keep up with regular water changes, remove the dead fish and plant matter from the tank as quickly as possible.  Leaving these items in the tank will certainly cause your phosphate levels to spike and create the green water that you see. 

In summary, if you clean out your filters, keep the lights on a timely schedule, change the water consistently, cut back on feeding, and avoid using phosphate-based pH additives, the green water should clear out within a week or so. 

Cloudy Fish Tank Caused by Aquarium Glass

Another major factor of cloudy fish tank water is the aquarium glass that you use.  Newly established tanks have a high level of dissolved constituents, such as phosphates, silicates, or heavy metals.  A simple test of the water will likely show a high pH level.  There are conditioners for the tank that you can buy that will easily solve this issue.  You can also use RO (reverse osmosis) water.  Local fish shops are usually the best place to go if you are looking to use reverse osmosis water, as they tend to sell the water itself or units capable of making reverse osmosis water. 

Other people have been so bold as to use a razor or steel wool on the new glass to scrape off the excess constituents.  Steel and wire wool is often more highly recommended because the steel wool does not scratch the glass when you are trying to clean it. 

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