Why is my aquarium water cloudy?


Cloudy water can be worrying for a new aquarist, and outright alarming for an established one. So, what causes your aquarium water to become cloudy and how can it be fixed?

Fishkeepers soon learn that the closed aquarium is a finely balanced ecosystem, and that even minor changes to water chemistry can have major consequences. Many of these changes can be harmful, but unseen and require testing, though some of them can cause discolouration to the water as well. In other cases, cloudiness or a change in the water’s colour can be quite normal and harmless. 
Broadly speaking, cloudy or discoloured water can be divided into a few different types: biological, material, algal, chemical, and gaseous. Here’s what you need to know about each. 

Reasons for a cloudy aquarium

Material cloudy water

Perhaps the most common cause of cloudy water is from particles suspended in the water. These remain in suspension until the filter is able to remove them or the supply of particles is exhausted. It is especially common in brand new aquaria directly after filling with water. 

Common causes of material cloudy water

1. Unwashed sand or gravel

Sand and gravel is often notoriously dirty and requires a thorough clean before going into the aquarium. This can involve multiple rinses of fresh water, usually carried out repeatedly in a bucket until the water runs clear. Usually, unwashed substrates result in a white, powdery cloudiness, although in the case of coloured substrates, a giveaway can be if the cloudiness is the same colour as the new sand or gravel.
A low level of cloudiness from new substrates is quite normal, and will normally pass within a day or two as the filter sifts out the particles from the water. If the tank is so cloudy that it looks as though it has been filled with milk, it will require a large water change to try to dilute it back down. In the worst cases (if intense cloudiness persists for over a week), you may need to remove the substrate and clean it properly before adding it back to the tank.
With a low level of particulate cloudiness, a flocculant treatment such as Seachem Clarity can be used. This will clump the small particles together into a size that’s easier for the filter to remove. 

 2. Unwashed filter media

In a new filter, the biological media, as well as any carbon, is incredibly dusty and needs to be thoroughly rinsed before first being used. Simply running the new media under a coldwater tap until the water runs clean will do this. (Note that you should never rinse established filter media under a coldwater tap as this will kill off any beneficial bacteria.)
As with dirty substrates, a flocculant treatment can be used to speed the clearing process.

3. Lack of fine filter pads

One role of the aquarium filter is to remove small particles from the water (we call this mechanical filtration). If the filter system doesn’t contain anything that can trap out fine particles, then the action of the filter pumping water around the tank will keep these particles in suspension indefinitely. This is rectified by simply adding some fine foam pads or filter wool to your filter. 

4. Digging fish

Some fish, especially many catfish and eartheater cichlids, routinely dig and grub about in the substrate. If the substrate is especially fine or silty, this can result in particles being repeatedly sent up into the water column. However, this only tends to create a drastic problem if the substrate hasn’t been appropriately cleaned before the fish were added. 
If it’s only a minor amount of clouding and you have a tank full of digging fish, it’s something that you’ll have to learn to live with. If it causes extreme clouding, you may need to consider a different (cleaner) substrate, though this is incredibly rare.

Biological related cloudy water

The biological aspect of filtration relies on bacteria to control fish wastes. If these bacteria are upset then the water can soon appear cloudy. But other biological causes can also discolour water. 

Common causes of biological related cloudy water

1. Unestablished filter

Newly set up tanks can require considerable time to become biologically mature—the bacteria that convert harmful fish wastes to less harmful ones can take weeks or even months to fully establish. Specifically, we want something called autotrophic bacteria inside our filters. If these bacteria are overwhelmed (especially if too many fish have been added to the tank before they’ve established) then competing heterotrophic bacteria can appear to exploit the waste in the water. These bacteria will make the water appear cloudy and white for as long as the pollution persists. During such a bloom, it is also likely that the water will test for elevated levels of ammonia and nitrite. 
It's not just new tanks that can become cloudy because of a filter issue. If the filter bacteria become compromised in any way (if the filter media is cleaned incorrectly, or if the bacteria are impaired by a course of fish medication) then such cloudiness can occur in an otherwise established tank.
To treat this type of clouding, the bacteria will need to be supplemented inside the filter. Something like Nitrico Goop, or one of the many liquid biological boosters available, will do the job. It is also worth considering additional aeration in the tank, such as from an airstone connected to an airpump, as the bacterial bloom will deplete oxygen levels. 

2. Old tank syndrome

If a tank has been running for a long time (usually many years) without adequate maintenance (especially adequate water changes) then the bacteria in the filter may exhaust the tank’s supply of carbonates. This in turn stops the bacteria from converting waste properly, and in turn causes the water chemistry to fall out of equilibrium, leading to (amongst the things such as a pH crash) a sudden outbreak of white, milky cloudiness. 
While it’s essential to re-establish the biological filter using something like Nitrico Goop, it’s also vital to address the underlying problem. The tank will need several partial waterchanges (around 25% a day over the course of a week), and perhaps even a hardness buffer to bring the carbonate levels back up. 

3. Overfeeding

Too much food into an aquarium will put extra burden on the filter, especially if much of that food is uneaten. As with an unestablished filter, if there’s more pollution than the bacteria in the filter can handle, other types of bacteria will start to grow to exploit it, leading to cloudiness. 
The solution here is clear. Address how much food the fish are receiving, and carry out a series of small waterchanges (using a gravel cleaner to get all the uneaten food out from the substrate) until the problem is resolved. 

4. Dead fish or plants

As with uneaten food, a dead plant or fish will produce a huge surge of pollution, putting pressure on the filter and leading to a potential bloom of opportunistic bacteria in the water. Remove the offending plant or fish, and carry out series of water changes, or a large (50%) waterchange, pending how high ammonia and nitrite levels are. 

5. Natural decoration

Many aquarium decorations are natural in origin, and these are made up of organic matter that can cause bacterial blooms. Some aquarium woods can cause water cloudiness soon after being added to a tank (as well as developing slimy-looking fungal growth over them) though these will typically pass within a few days. Many botanicals such as leaves and seed pods are rich in nutrients that encourage a bloom of bacteria in the first few days after adding them. These are nothing to worry about and will also pass quickly. 

Algae related cloudy water 

Algae is the blight of almost every aquarist, but usually it stays relatively manageable by clinging to glass and decoration. When it goes rogue and waterborne, it can be a real menace. Worse still, it can seemingly come out of nowhere.

Common causes of algae related cloudy water

1. Excess light and nutrients

Like other plants, algae need three things to flourish: light, food, and a carbon source. In heavily stocked, brightly lit tanks, algae may suddenly develop directly in the water, leading to a green tinge that can range from pale to pea soup coloured. This problem tends to affect planted aquascapes more than generic fish tanks, and can be linked to incorrect amounts of plant food being added to a system. Because algae are single-celled organisms, they are typically too small to be trapped within filters, meaning that they just get churned around the tank.
Tanks blighted with heavy infestations of green water run the additional danger of becoming hazardous to livestock. So much suspended algae can play havoc with oxygen levels, carbon dioxide and pH, and needs to be addressed quickly.  
Controlling it can be difficult. In many cases, waterchanges may only be a temporary fix and encourage the green water to return even stronger than it was before as new nutrients are introduce into the tank.  
Adding chemicals like King British Green Water Control can help by flocculating the algae (sticking the individual cells together so that they sink or get trapped within the filter) after which the aquarist can use a gravel cleaner to remove the clumped algae from the bottom of the tank and clean it out of filters. 
For long term control, the best method is to install an ultraviolet clarifier (UVC) like the Fluval UVC In-Line Clarifier. These devices work in tandem with an external canister filter that pushes water through them. Inside a UVC is an ultraviolet light that causes the algae cells to corrupt and become sticky. These sticky cells then clump together and become trapped inside filters where, denied access to light, they die. 

Chemical related cloudy water 

Some of the things we add to water to alter its chemistry can cause cloudiness. However, these are almost always due to user error. 

Common causes of chemical related cloudy water

1. Medication overdose 

Many of the fish medicines we use are antimicrobial, killing parasites, fungi and bacteria. Sadly, not all of them discriminate between pathogenic microbes and the beneficial microbes inside an aquarium filter. When overdosed, any medication has the potential to wipe out filter bacteria, resulting in a build-up of pollution and subsequent cloudiness. 
Controlling cloudiness from a medicine overdose involves removing the excess medication from the water through a combination of waterchanges and the addition of activated carbon. With the medication now removed, the biological filter will need to be re-established using something like Nitrico Goop. 

2. Incorrect buffer dosing 

Salts to increase water hardness or elevate the pH of water need to be pre-mixed prior to adding them to a tank. If the powders are added directly to the tank, it’s quite common for them to create a whitish precipitate and cloud water (as well as causing dusting over substrates and decoration). 
Controlling this requires an immediate partial water change and gravel siphon to remove as much of the precipitate as possible. Further episodes can be avoided by mixing any buffers prior to adding them to the tank. 

Gas related cloudy water 

Sometimes cloudy water can be the result of suspended gases. In rare cases, these are harmless, and instead tend to be a sign of a major problem inbound. 

Common causes of gas related cloudy water

1. Filling a tank with cold water

Immediately after filling a tank for the first time, it’s common to see cloudiness in the water, combined with with the formation of tiny bubbles on the glass. This is a natural phenomenon of dissolved gases in water—cooler water has a greater ability to contain dissolved gases than warmer water. As the water warms, the gases start to form tiny bubbles. (You’ll see a similar thing occur if you take a cold glass of water to bed at night. By morning it will be full of tiny bubbles.) This will make the tank hazy for up to 24 hours, though usually less. 
As this is entirely normal in a new tank, it is nothing to worry about. But it should be mentioned that for this reason, you should never fill a tank with cold water after a waterchange. 

2. Loose connections

This is a problem that tends to affects tanks with external filtration (external canisters or sumps) rather than tanks with internal filters. Where water is passed at speed through lengths of tubing, if there are any tiny leaks or fissures, the rushing water may also suck in a fine misting of bubbles. The bubbles cause supersaturation of gases in the water, which can then lead to embolisms in fish. These may cause bubbles to form in the skin, or build up behind the eye, swelling and pushing the eye out. The water itself maybe clouded with tiny bubbles, or may have a vague, white misting to it. 
The cure for this involves systematically going through every connection from tank to filter and back again to make sure that they are properly sealed and airtight. Seal rings in canisters should also be checked for damage and replaced where needed, but these are less commonly the issue.