How to Fix Cloudy Water in a New & Existing Tank

One of the most common questions both experienced and new aquarium owners ask is how to fix cloudy water in a fish tank. So we have decided to create this update to one of our most popular blog posts "How To Fix Cloudy In a New Fish Tank" to include addressing cloudy water in an existing tank in this ultimate cloudy water guide.

The good news is that cloudy water in an aquarium isn't necessarily an emergency. In fact, there are some very simple ways to diagnose and treat cloudy water, depending on whether you're working with a new or established aquarium and the color of the water.

In this guide, we'll explore...

  • How to Fix Cloudy Water in a New Tank
  • How to Fix Cloudy Water in an Existing Tank

How to Fix Cloudy Water in a New Tank

Does your brand-new aquarium suddenly have cloudy water after being clear for the first few days? Don't worry. A newly set up aquarium is a biological blank slate. There are virtually no life forms present on Day One. For a quick overview check out our infographic on fixing clouding water in a new tank.

Within days of first setting up your tank, a variety of microscopic organisms will try to establish themselves in the aquarium's water. The beneficial nitrifying bacteria that filter the water and create balance haven't had a chance to colonize yet.

This means that for a week or so, it's kind of a free-for-all. Free-floating bacteria and other microbes take advantage of minerals and nutrients in the water and begin to multiply unchecked. This is what can cause cloudiness.

However, the situation is sometimes made worse when hobbyists add too many fish at once or overfeed their fish. Either can provide microbes with an additional food source.

So, just how many fish should you have in your aquarium? A good starting point is to aim for roughly one inch of adult fish for every net gallon of aquarium. Keep in mind that if you add a lot of decor, you may want to have fewer fish than this to account for less swimming room. And fish that tend to be more territorial may need more space than this.

So if you see cloudy water in a new aquarium, it's best to just let it run its course. The most effective measures are preventative ones that need to be done before the cloudiness kicks in

Here are nine helpful tips you'll want to follow for a new aquarium to help keep the water clear. You can also find a quick guide on kickstarting your new aquarium in the Aqueon 3 Tips to Succeed video.

1. If Your New Aquarium Water's Cloudy, Just Let Nature Take Its Course

Without question, doing nothing is the best approach for a new fish tank as long as ammonia and nitrite levels are not on the rise. Cleaning the filter does nothing except disrupt the few beneficial bacteria that have had a chance to get established. These "good guys" will eventually out-compete the cloudy water bacteria for food.

Water changes may temporarily clear things up, but in a day or two, the cloudiness will come back, often worse than before. That's because new water provides a fresh supply of nutrients, causing the cloudy water bacteria to populate even more.

Left alone, the cloudy water bacteria will eventually consume all the nutrients in the water and die out. This is part of the cycling process!

2. Add Live Plants to Your Aquarium

Live plants have "good" bacteria and other microbes, which help establish the biological balance in the aquarium.

They also compete for nutrients and help starve out microbes that cause cloudy water. In addition, they produce oxygen during the day, which aids in the breakdown of fish waste, uneaten food, and even the cloudy water bacteria itself as it dies off. This also helps clear the water.

Additionally, plants consume ammonia generated by fish and uneaten food, which can build up until the good bacteria become established.

3. Don't Get a New Filter

When dealing with "New Tank Syndrome" cloudy water, an important rule is: Don't mess with the filter — no matter how much you're tempted to do so!

Cleaning a brand new filter or replacing the cartridge does nothing good and potentially eliminates the good bacteria trying to get established. If the filter pad or media needs cleaning before the first 30 days, this is a sign of a different problem. You may be overfeeding, overstocking, or both.

4. Don't Change the Water More Often

Regular, partial water changes are the #1 thing aquarists should do to be successful, except during the first 30 days. As mentioned above, water changes may help clear the water temporarily (24 hours at best), but the cloudiness comes back with a vengeance because you have given it a boost of nutrients with the incoming water.

5. Don't Overfeed Your Fish

Beginning aquarists often fear their fish will starve to death, so they feed heavily and often. Unfortunately, there are few, if any, nitrifying bacteria present to break down the resulting waste or uneaten food. So only feed what your fish can eat in about two minutes, just once or twice a day.

When there's a lot of uneaten food, the cloudy water bacteria take advantage of this and continue to multiply. Even worse, harmful ammonia and nitrite levels may begin to rise. Fish in nature don't always eat every day. Sometimes they might not eat for days, and other times if they find a lot of food, they might eat several times. Because they are naturally opportunity feeders, they might appear to "beg" for food they don't really need. So just stick with the once- or twice-a-day feeding schedule.

6. Don't Put Too Many Fish in Your Tank

More fish means more waste and more food for the microbes causing the cloudy water. Too many fish in a new aquarium may also cause a rise in harmful ammonia and nitrites.

7. Add Activated Carbon Media to the Filter

Adding activated carbon media cartridges, or activated carbon pads to the filter will help clear the water and absorb nutrients that feed the bacteria bloom.

8. Seed the Aquarium

If you have access to another healthy, well-established fish tank, adding a few handfuls of gravel from that aquarium will seed the beneficial bacteria and speed up the clearing process.

Aquatic stores sometimes keep filter cartridges, bio-sponges, and wheels floating in stocked aquariums to seed them with bacteria and will send these items home with new setups to help get the biological balance going. This has the same effect as adding gravel from an established tank.

You could also try a biological starter product like Aqueon PURE. These contain cultures of beneficial bacteria and can help the cycling process. With it's unique "time-release" design, you can't overdose your tank environment with the bacteria.

9. Test Your Aquarium Water

Test aquarium water for ammonia and nitrite as soon as the water begins to cloud. (You can typically get a test kit at any fish store.) In most situations, the levels will be zero, meaning there is no cause for concern.

Seeing cloudy water in a new aquarium can be alarming. But the best advice is to be patient and wait it out. Don't add any more fish, feed sparingly once every other day, test your water, and just leave the filter alone for the time being. It should all sort itself out.

How to Fix Cloudy Water in an Existing Tank

Cloudy water in an established aquarium is another issue and won't typically go away if left alone. Sometimes it's caused by an overgrowth of bacteria suspended in the water, which is referred to as a bacterial bloom. But there are a variety of possible causes. For a quick overview check out our infographic on fixing clouding water in a existing tank

There are even different types of cloudy water with separate solutions. Is your cloudy water white or gray, yellow or brown, or green? We have sections below identifying the causes of each type, along with steps you can take to help alleviate the issue.

1. White or Gray Cloudy Water

Water that's turned a milky white or gray in an established aquarium is typically caused by bacterial overgrowth, commonly the heterotrophic variety. These break down fish waste, uneaten food, and the like. So the leading causes generally are dead fish, plants or excess leftover food in the tank from overfeeding.

The biggest danger to fish during this bacterial bloom is not having enough oxygen, so it's a good idea to increase aeration until you've fixed the issue.

How to Fix White or Gray Water

Cleaning your aquarium is typically enough to solve the problem. Just do your regular partial water change, and vacuum the gravel to get rid of decaying debris or uneaten food. You might also need to clean the filter to make sure it's working at peak efficiency.

If it doesn't clear up, you can try adding flocculants (aka the Aqueon Water Clarifier), which cause debris to clump together, making it easier to remove.

Preventing Another Outbreak

Of course, fixing the problem won't do much if you don't stop what caused it. So don't let anything build up that the bacteria like to live off. This means cleaning your tank regularly, which can help eliminate waste. And don't overfeed your fish. Just feed them what they can eat in two minutes or less, twice a day at the most.

2. Yellow or Brown Cloudy Water

Different things can cause cloudy water that has a yellow or brown tint. Sometimes it's caused by decaying organic matter. You can usually see decayed plants, dead fish, or uneaten food in the tank, causing the problem. The water might even look a bit foamy.

If you're maintaining a planted aquarium, the discoloration could be caused by natural foliage which contains tannins. Driftwood, which is basically natural wood that you can buy at a pet store, is often a common source of tannins. Although it also refers to wood that naturally washed ashore, it's safest to use pieces specifically sold for aquariums to protect your tank environment

Tannins refer to tannic acid, which is released as driftwood decays. This can cause the water to turn yellow or brown and reduce pH levels. The good news is that tannins aren't dangerous to your fish. In fact, sometimes people even allow tannins to create a more natural environment for certain fish.

However, sometimes water can turn yellow or cloudy because ammonia or nitrite levels are too high. This can be dangerous for your fish, so you'll need to test your water. You can get a water test kit at your local pet store. Ideally, your aquarium won't have any nitrite or ammonia levels, but your test kit will tell you if and how dangerous your levels are. Follow the directions closely to ensure your test is accurate.

How to Fix Yellow or Brown Water

If the cause is decaying matter, cleaning your aquarium can fix the issue. Remove any decaying plants or fish, check the filter, and use a vacuum to clean the gravel. You don't need to change out a lot of water; your typical amount of 25% every month or so is just fine.

If tannins are causing the discoloration, then adding aquarium-activated carbon to your filter is one of the fastest ways to reduce the yellow in your water. Why? Because carbon removes tannins. Just read the directions, and, if you're using any medications for your fish, make sure the carbon doesn't interfere with the medicine. The Replacement Filter Cartridges from Aqueon, for example, come with 25% more activated carbon than many other brands.

If tannins are the cause, simply changing the water regularly can also help, but that may take a few months.

Now, if the problem is ammonia and nitrite, this can be more serious. If ammonia levels are high, fish can suffer internal organ and gill damage or even death. High nitrite levels can make it tougher for the fish's blood to carry oxygen, leading to suffocation dangers.

Change out about 50% of the water, then test the ammonia and nitrite levels again. Don't go overboard and clean everything, like the decor and plants. Do a little bit at a time to preserve your helpful bacteria colonies.

Prevent Another Outbreak

One way to reduce tannins in your water is by taking steps before the cloudiness starts. For example, soak driftwood in hot water for at least a few hours before you add it to the aquarium. It's important to note that you can never remove tannins completely from driftwood, but you can reduce the amount so it's not as obvious in the water.

High levels of ammonia can be caused by too much decaying matter, bacteria buildup, or chemically treated tap water. Make sure you're only using water that's been conditioned for your aquarium.

Having a working, clean filter and regularly cleaning your tank and changing it's water can help prevent the buildup that feeds ammonia. You'll also want to ensure your filter is big enough for your tank (based on the filter's instructions) and that you're not overfeeding or overstocking your tank. Use an aquarium test kit once a week to test your ammonia and nitrite levels, which ideally will be close to zero.

3. Green Cloudy Water

Cloudy water that's green in color is often caused by algae overgrowth

More specifically, it's caused by an overgrowth of phytoplankton that floats in the water and can multiply fast. But knowing the type of algae isn't enough. You'll need to step back and figure out what caused the algae to grow so much in the first place.

Potential culprits come down to one of two possibilities:

  1. Too much light is allowing your algae to thrive.
  2. You have too many nutrients like phosphates and nitrates that algae feed off. (Note: These are nitrates, which aren't as dangerous as the nitrites mentioned in the yellow water section.)

How to Clear Up Green Water

Install a UV sterilizer to clear up green water in a few days. These are safe for fish, plants, and other tankmates while eliminating suspended algae that can cause cloudy water. You can purchase UV sterilizers at pet or fish stores.

You can also try keeping the light off completely for two to three days and see if that gets rid of the algae. However, this is more of a temporary patch since it doesn't address the cause (unless the cause is light).

As a last resort, you might try chemicals that get rid of green water blooms, like the Algae Remover from Aqueon. Be sure to read the directions closely. Don't use the Aqueon Algae Remover if you have shrimp, lobsters, or crabs. Like turning off the lights, this type of product addresses the symptoms, but it doesn't fix what caused the bloom in the first place.

Prevent Another Outbreak

Whatever approach you use, you'll want to prevent another outbreak in the future. These steps can help.

First, install a timer, so the lights aren't on too long. Aquariums with live plants may need eight to 12 hours of light a day, but aquariums without plants can handle six hours or less.

Regularly test your aquarium and tap water's nitrate and phosphate levels. Just buy a water test kit at your local fish store and follow the instructions closely. If the tap water levels are high, consider using reverse osmosis or deionized water with a Water Renewal product, like the Aqueon Water Renewals.

Change about 25% of your water a month, or less if changing weekly. Use a gravel vacuum to get out excess waste or food, and change out your filter according to its instructions.

As mentioned before, make sure you don't have too many fish producing excess waste for nitrates to feed off. And don't overfeed them!


As you can see, there's no reason to panic if your tank water's cloudy. If you have a new tank, your best course of action is to just wait it out. If your tank is established, your course of action will depend on the color of the water. And don't worry: learning the ups and downs of cloudy water is just part of being an aquarium owner!