There are a variety of reasons your aquarium water can be cloudy, but they all come down to 4 primary causes:

1. Bacterial Blooms
2. Algae Blooms
3. Suspended Particles
4. Dissolved Compounds

Once you know what is causing your cloudy water, you can take steps to clear it again.


What are bacterial blooms?
Bacterial blooms are caused by an overabundance of bacteria in the aquarium water column. Bacterial blooms usually look like a milky-white cloud in your tank. If the bloom is severe, you may even see white, slimy strings of bacteria on the surfaces of the glass or aquarium hardscape.

What causes bacterial blooms?
Bacterial blooms happen when there is a sudden increase in the amount of nutrients in the water. The bacteria grow quickly in order to eat and break down the excess nutrients. Usually, these nutrients are in the form of ammonia and nitrite. While the bacterial bloom is ugly, it is a sign of a deeper problem. High ammonia and nitrite can kill fish and other aquatic animals.

What to do about bacterial blooms?
If you think you have a bacterial bloom, start by testing your water for ammonia and nitrite. There should be no detectable ammonia and nitrite in an established aquarium. If your tank is new, then a bacterial bloom is normal and should pass in a couple of days to a week. Your aquarium is establishing a healthy bacterial population that will help keep your aquarium water clean and clear in the long run. If your aquarium is more than a month old and already established, and you see ammonia and nitrite, perform a partial water change as soon as possible to get the levels down and help save your fish. Ammonia and nitrite are highly toxic to fish. Once the levels are under control, try to identify what caused them to rise. A dead fish, overfeeding, or a malfunctioning filter can all lead to excess nutrients that can trigger a bacterial bloom.

What if it keeps happening?
If you have continual bacterial blooms, then you are likely overfeeding or under-filtering your aquarium. You will need to either reduce the amount of fish food you are feeding or increase filtration to compensate. Adding a UV sterilizer will stop bacterial blooms by killing bacteria in the water, but it will not address the underlying problem of toxic ammonia and nitrite levels. Before you purchase a UV sterilizer, first make sure you have ammonia and nitrite levels under control.

Further Reading:
-UV Sterilizer Overview
-Aquarium Testing Recommendations


What is an algae bloom?
Algae blooms occur when there is a sudden increase in free-floating algae. This typically looks like cloudy green aquarium water. This is also very common in outdoor ponds. If you have seen a pond with green-tinted water, you have seen an algae bloom.

What causes algae blooms?
Algae blooms are caused by a combination of light and nutrients. Just like with bacterial blooms, an abundance of nutrients can lead to an algae bloom. What often causes an algae bloom rather than a bacterial bloom is the amount of light. If there is low light and a sudden influx of nutrients, a bacterial bloom is likely; if the aquarium is brightly lit, an algae bloom is more likely. Algae blooms can also occur when light levels suddenly increase. This can happen when you leave your aquarium light on overnight, let the sun shine on the tank all day, or switch to a more powerful light.

What to do about Algae Blooms?
Start by testing your aquarium water for ammonia and nitrite. If either of these levels is high, perform an immediate partial water change, as the aquarium water is unsafe for fish. After the water change, try to identify what caused the nutrients to spike. It could be a dead fish, overfeeding, or a malfunctioning filter. If you test the water and detect no ammonia or nitrite, excess light is likely the cause of the algae bloom. Make sure that your lights are only on for 8-12 hours per day. Keep the aquarium out of direct sunlight, and if it has dimmable lights, turn them down to lower the light levels. In some cases, you will not be able to lower the light levels, such as when the aquarium is in a brightly lit room, next to windows without window coverings, or outdoors. In these cases, adding a UV sterilizer is the best option. The UV sterilizer will kill algae in the water column, keeping the tank clear.

What if it keeps happening?
If you have continual algae blooms, then adding a UV sterilizer is the best option. UV sterilizers are nearly a necessity for ponds and outdoor aquariums because of the access light.

Further Reading:
-UV Sterilizer Overview
-Aquarium Testing Recommendations



What are suspended particles?
Suspended particles are simply small solids that are floating around in the aquarium water. They can come from uneaten fish food, aquarium substrate, fish poop, fish and invertebrate spawning, and many other things.

What causes suspended particles?
There will always be some suspended particles in the aquarium water. Some particles can be beneficial, such as fish food or plankton. However, they can get out of hand and ruin the viewing experience. Some common causes of excess particles include freshly added aquarium gravel, burrowing fish or animals that stir up sand, and spawning fish or invertebrates.

What to do about suspended particles?
The best way to remove particles from the aquarium water is with mechanical filtration. The finer the mechanical filtration, the smaller the particles it can remove. The gold standard for mechanical filtration is a 25-micron filter. 25 microns is the smallest particle size the human eye can see, so these filters will remove anything that you could see as a particle, while not being so fine that they quickly clog. If your tank is cloudy due to a temporary event, such as adding new sand, using a polishing filter, such as the Marineland Magnum 200 Polishing Internal Canister Filter, can quickly clear up the water. You can remove the filter once the water is clear. These filters use 25-micron cartridges and will remove any particles you can see.

What if the particles keep coming back?
If you constantly have particles in the water, it may be best to add a permanent polishing filter. The best option is the Inland Seas Nu-Clear Model 533 Mechanical Canister Filter, 25 Micron Cartridge. This is a canister filter that uses a very large 25-micron cartridge to polish the water. Use the 1533 Extension for extra-large aquariums. NuClear Canister Filters are very popular in small commercial aquariums where crystal-clear water is important for viewing, but they can also be used for home aquariums. For more information, see our Nu-Clear Canister Filter Overview video.




What are dissolved compounds?
If you have seen a glass of tea, you will notice that the tea has a dark tint to it yet there are no particles in the tea. All of the tea leaves added to the water are fully dissolved. Aquarium water can also contain dissolved compounds from organic sources, such as fish foods, fish waste, driftwood, and plants. These compounds are fully dissolved, and no amount of mechanical filtration will effectively remove them from the water. Dissolved compounds often appear as a slight brown tint to the aquarium water. Often, you may only notice the tint against a white background or when looking through the short side of the aquarium, from the length of the aquarium to the other side.

What causes dissolved compounds?
Dissolved compounds come from fish foods, fish waste, plant waste, driftwood, coral excretions, and many other sources. If you notice a sudden increase in dissolved compounds, then some common causes are new fish foods or new driftwood. Dissolved compounds build up over time. In aquariums that do not receive regular water changes, the water may take on a significant brown tint.

What to do about dissolved compounds?
The simplest way to remove dissolved compounds from your water is with carbon. Inland Seas Activated Carbon is our favorite because it is low in dust and easy to use in most filters. Simply add some carbon to a media bag, rinse the carbon to remove dust, and then add the carbon bag to a high-flow area of the aquarium, such as a canister filter, sump, or hang-on filter. The carbon will absorb dissolved compounds, clarifying the water. In most cases, you will notice a water quality improvement within 24 hours. Carbon can only absorb so much before it loses its effectiveness. The vast majority of carbon’s usefulness is within the first week of use.

What if the dissolved compounds keep coming back?
It is normal for dissolved compounds to slowly accumulate in aquariums. Because of this, we recommend replacing carbon on a monthly basis. This will keep the dissolved compound levels low and lead to clearer, better-looking water.