Why Is My Fish Tank Cloudy – Causes, Colors and How To Fix It

A cloudy fish tank may raise alarm bells for some. We’d all love to have a tank with perfectly clear water to show off our colorful finned friends and their habitat’s aesthetic decor. More importantly, we want to ensure that our beloved fish are safe and healthy.

Freshwater tanks go cloudy for a variety of reasons such as algae and bacterial bloom or residue from substrates and accessories. Many of these factors can be easily overcome with proper tank maintenance, frequent water testing and a little bit of patience.

Read on to learn more about why your tank may be cloudy, what the different cloudiness colors mean, and how to fix and prevent it!

Here’s a TLDR infographic to summarize what we covered!

Infographic - Why is my fish tank cloudy and how to fix it

White or Grayish Cloudiness in Fish Tank

Cloudy fish tank water with white discoloration
 Is it harmful?

It depends. If caused by dirty substrates, it’s harmless. But cloudiness may indicate unwanted bacterial blooms which can diminish tank oxygen levels.

It’s quite common to find white or grayish clouds in your tank’s water, especially for new aquariums or after cleaning or cycling the water. This may be caused by impurities in the substrates added or by a rapid growth of bacteria in the system.

Residue from New Substrate

If you’ve just gotten a new bag of substrate, it may be the culprit for the white or gray cloudiness in your tank. 

Gravel, sand and crushed coral substrates tend to accumulate dust build-up after being packed and transported to stores. Once added to your tank, the dust plumes up and causes the water to go cloudy.

 How to fix it:

The best way to prevent this is to thoroughly rinse any substrate before using it. Here are the steps:

1. Pour your substrate into a bucket and fill it with water
2. Run your hand through the mixture several times
3. Pour the dirty water out
4. Repeat the process until the water appears clear

Your substrate should be ready for use after 24 hours of drying.

You may also:

  • Do a 50% water change to try to get rid of the dust
  • Use a super fine mechanical filter media or polishing pads to trap the impurities
  • Leave it be — the tank’s filtration system should clear the dust after a few hours or days

Rapid Growth of Bacteria

New Aquarium

First-time aquarists may be surprised when their brand new tank turns cloudy even without fish in it. This isn’t a cause for concern. In fact, it’s a good sign that healthy bacteria is growing rapidly within your tank, creating a suitable living environment for your fish. 

These microorganisms are important for oxidizing ammonia and nitrite, removing these toxic substances from your tank.

 How to fix it:

It’ll take about a week or so for the bacteria and microbe growth to stabilize naturally. Leaving your tank to balance itself out in that time should cause the cloudiness to disappear.

Addition of New Fishes

Pet fishes in fish tank

You may find that your tank turns cloudy all of a sudden when increasing your fish population. This is because the addition of new fishes acts as a new source of nutrients and waste for the nitrifying bacteria in your tank. The new additions create an imbalance in the water, causing bacteria to bloom and stabilize it.

This bacterial bloom causes the white and grayish cloudiness in the tank.

 How to fix it:
  • Leave your tank to restabilize itself for 1 to 2 days
  • Do a 25% to 40% water change every few days while monitoring cloudiness levels if the problem persists
  • Don’t add any new fish during this time

To avoid this when populating your tank, you should aim to add less than 50% of the current population at a time. 

Let’s say you currently have 6 fishes in your aquarium and want to introduce 6 new ones. You should only introduce 1 or 2 new ones at a time over a span of 4 to 6 weeks.

Rebuilding of Bacteria Colony

Similar to the cloudiness found in new aquariums, your tank water may become discolored if bacterial colonies are rebuilding themselves. This is triggered when your existing colony is eradicated by the chlorine in untreated tap water or by medications like antibiotics.

 How to fix it:

You should give your tank a few days to restabilize itself, allowing nitrifying bacteria to recolonize the environment.

For prevention, it’s recommended that you use a water conditioner every time you change the water, especially when using tap water. When rinsing and cleaning filter media, use existing tank water instead of tap water to retain healthy bacteria.

Filtration System Breakdown

Cloudiness may also be an indication that there’s a problem with your filters. A low quality filter may not clear off debris and waste too well, causing your water to be dirty. They also don’t do a good job of clearing nitrites and ammonia, which gives bacteria an excess of nutrients to consume. This may lead to bacterial bloom.

 How to fix it:

Ensure that your filtration system is working properly and replace any faulty or damaged parts. Make sure to keep your filters clean too, as this can affect their effectiveness. Once the system is up and running again, allow the bacterial bloom to run its course.

Green Cloudiness in Fish Tank

Cloudy fish tank water with algae and green discoloration
 How to fix it:

It can be. Green cloudiness usually indicates high algae levels, which can compete for oxygen with your fish.

Green discoloration isn’t just unsightly, it’s also an indication of algae bloom in your tank. While algae won’t directly hurt your fish, a high presence of the microorganism can begin to deplete tank oxygen levels. It’s good to resolve or prevent the issue before it begins affecting your fish. 

Algae blooms usually happen when there’s an excessive amount of light or nutrients in the tank.

Excessive Light

Leaving your tank’s light on for too long or exposing it to intense amounts of light causes algae to thrive excessively. This is because the algae cells in your tank, Euglena, produce food for themselves through photosynthesis.

 How to fix it:

Minimize light usage in your aquarium until the algae levels start to deplete. Ideally, you should only leave your tank light on for 8 to 10 hours a day.

Excessive Nutrients

Algae also thrives off nutrients in the tank, such as nitrates and phosphates. These nutrients build up when:

  • Water isn’t changed regularly enough
  • Dead fish and plants are left in the tank
  • There are too many fishes in the tank
  • You’re overfeeding your fishes
 How to fix it:

Use a diatomic or micron filter to get the algae cells out of the water. Alternatively, they can be killed using a UV sterilizer installed in the tank. 

You should also avoid overfeeding and overstocking your fish to keep nitrate and phosphate levels low. Introducing more plants into your tank can help to do this as well.

Brown or Yellowish Cloudiness in Fish Tank

Fishes swimming in a fish tank with brown yellowish cloudiness
 Is it harmful?

Typically, no. It’s unsightly but it doesn’t affect your fish.

Brown discoloration is typically caused by a harmless organic substance called tannin. This substance is quite abundant in nature, commonly found on tree barks and plants. Tannic acid, a type of tannin, is usually introduced into aquariums by botanicals and driftwood.


When dissolved, plants and wooden decor release tannins into the water all the time. Over time, this may turn your water brown if you don’t do regular water changes. 

Your water may also suddenly turn brown if you’ve recently added unsoaked driftwood into your aquarium.

A quick way to know if tannic acid is discoloring your water is to use a test kit. Measure the tank’s pH levels using the kit — the acid may lower the pH slightly.

 How to fix it:

You can use chemical filters like carbon to get your water crystal clear again.

It’s also good to soak driftwood decor for 3 to 7 days, replacing the water each time it gets murky, before adding it to your tank.


 New Tank Syndrome
New tank syndrome refers to the sharp rising of toxic nitrite and ammonia levels in new aquariums, causing the death of fishes. The water in a new tank takes some time to build up the nitrifying bacteria that gets rid of these toxic ions. This process can take anywhere from 2 weeks to 3 months, depending on how you do it.

There are several methods to overcome new tank syndrome, such as:
  • Seeding
  • Slow cycling
  • Making use of cycling bacteria

  • Whichever method you use, it’s important to keep an eye out for ammonia and nitrite levels. Use a test kit to check that these are kept under 1ppm and 1.5ppm respectively. You shouldn’t do water changes unless those levels have been exceeded.

    You should also avoid:
  • Replacing your filter and its cartridge or media
  • Overfeeding
  • Overstuffing

  • How To Prevent Cloudiness in Fish Tank

    Two goldfishes swimming in fish tank

    Preventing cloudiness is largely about maintaining balance in your tank. You don’t want excessive microorganism blooms, nor do you want to strip off all healthy bacteria colonies. Keeping excess nutrients to a minimum is the key to keeping your bacteria and algae levels in check. 

    To do so, you should limit their access to food and light, while keeping your tank water clean. Here are some tips on how to do just that!

    Regular Water Changes

    To keep your tank well maintained, it’s advisable to do a water change every 1 to 4 weeks. 

    • Lightly stocked tanks: 10% to 15% water change every 2 to 4 weeks
    • Heavily stocked tanks: 20% to 25% water change every week

    When dealing with a cloudy tank, many hobbyists do more regular water changes. You may try 25% to 40% water changes a week, or for nutrient spikes, every other day. Avoid 50% water changes where possible as it may strip your tank of nitrifying bacteria.

    Clean The Fish Tank Glass

    Red Easy Scrape Handheld Algae Scraper for fish tanks

    Sometimes, your aquarium’s glass may be the cause of the tank cloudiness. This is because bacteria and algae may form a biofilm on the glass, diminishing its clarity. It’s best to clean the tank walls at least once a month to keep it clear. 

    • Outer tank walls: White vinegar works best to get rid of water stains
    • Inner tank walls: Use brushes and cleaners like the Easy Scrape Handheld Algae Scraper

    Maintain Filtration System

    Fish tank filter cleaning tool – Marina Aquarium Brush Kit

    You should clean out your filters about once a month, clearing any solid build-up formed on it or its components. For any rinsing, remember to use tank water as you don’t want to remove all the nitrifying bacterial colonies on your filters.

    • Rinse and squeeze mechanical filter media to get rid of sludge and replace your filter floss once a week
    • Clean the piping and tubing in power and canister filters using tools like the Marina Aquarium Brush Kit

    It’s also a good idea to check the manufacturer’s recommended cleaning methods to properly maintain specific filter models.

    Fish tank water filter – HAILEA Internal Filter BT1000

    There’s also a bunch of filters and filter media in the market like the HAILEA Internal Filter BT1000. 

    Getting the most effective one for your tank may take some research and trial-and-error. But once you’ve got a good set up going, you’ll be spending less time cleaning up and more time admiring your gorgeous aquatic pets!

    Optimize Lighting

    T5 fluorescent light for fish tanks – Glo T5 Lighting System Double 24 Inch

    Too much light causes algae bloom, which turns your tank water into an unsightly green color. To prevent this, here’s what you can do:

    • Place your tank somewhere that avoids direct sunlight
    • Don’t leave aquarium lights on all day
    • Use a light timer to make sure their tanks aren’t exposed to too much light 

    You may also want to avoid using LED lights as it tends to create a dull, hazy look for your tank. T5 fluorescent lights are the preferred choice — try the Glo T5 Lighting System to keep your tank looking bright and lively!

    Prevent Overfeeding

    Automatic fish feeder – GEX Food Clock FC-002

    Overfeeding results in a build up of uneaten food that releases lots of nitrates and phosphates. 

    To prevent the cloudiness from bacterial and algae bloom, you’ll want to reduce organic waste as much as possible. Only feed fish what they’ll consume in about 30 seconds, or 2 to 3 minutes for slow eaters.

    Automatic fish feeders like the GEX Food Clock FC-002 are helpful for feeding precise amounts of food at adjustable intervals.

    Utilize Fish Tank Cleaners

    Fish tank cleaning solution – Microbe Lift Gravel & Substrate Cleaner - 8oz

    Many aquarists make use of fish tank cleaners at least once a month to keep their tanks in optimal conditions. 

    • Tank water: Use cleaning solutions like the Microbe Lift Gravel & Substrate Cleaner
    • Substrates: Use gravel vacuums
    • Tank wall glass: Use scrapers and magnetic cleaners

    FAQs About Cloudiness in Fish Tanks

    It varies, but cloudy aquarium water should take about 1 to 2 days to clear up. Sometimes, it may take up to a week for the water to get clear again. This duration may be dragged out if you accidentally remove healthy bacteria colonies during intervention.

    Depending on the cause, cloudy tank water typically won’t harm your fish, though it does point to an imbalance in their living environment. 

    Possibly harmful causes for cloudiness (if left unmediated for too long):

    • Bacterial bloom in unbalanced or new tanks
    • Algae bloom from excessive nutrients and light

    Non-harmful causes for cloudiness:

    • New substrates
    • Tannins from driftwood and plants

    Sometimes, water changes only manage to clear up aquarium water cloudiness temporarily. This is typically caused by healthy bacteria trying to rebuild their colonies in the water. 

    Water changes may even encourage this bloom by supplying more nutrients to the microorganisms. In these cases, it’s best to leave the water untouched and let the cloudiness dissipate on its own.


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