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. Even minor changes to water chemistry can have major consequences.
Many of these changes can be harmful, but are unseen and require testing. Equally, some of them can change the colour of the water. In other cases, cloudiness or a change in the water’s colour can be quite normal and harmless.
Generally, there are 5 main categories of cloudy or discoloured water: biological, material, algal, chemical, and gaseous. In this article we will examine each of these.
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. This is 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 dirty and requires a thorough clean before adding to the aquarium. This involves repeatedly rinsing the substrate with clean water in a bucket, until the water runs clear. Usually, unwashed substrates result in a white, powdery cloudiness. However, 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. This 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 is full of milk, it will require a large water change to try to dilute it back down.
In the worst cases, you may need to remove the substrate and clean it properly before adding it back to the tank. This is your best move if the cloudiness persists for more than a week.
With a low level of particulate cloudiness, a flocculant treatment such as Seachem Clarity is worth investing in.
2. Unwashed filter media
In a new filter, the biological media, as well as any carbon, is incredibly dusty. Rinsing them thoroughly before use is a sensible precaution. Note, you should never rinse established filter media under a tap as this will kill off any beneficial bacteria.
As with dirty substrates, a flocculant treatment can speed up 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 doesn’t contain anything that can trap fine particles, then these particles will continue floating in the tank. Simply add some fine polishing pads to your filter to ensure your filter is doing the job properly.
4. Digging fish
Some fish, especially many catfish and eartheater cichlids, routinely dig about in the substrate. This can result in these particles floating in the water column. This usually only creates a problem if the cleaning of the aquarium substrate was not sufficient to begin with.
If it’s only slightly cloudy, and you have a tank full of digging fish, you may just have to learn to tolerate it. If it causes extreme clouding, you may need to consider a different (cleaner) substrate.
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. New filters
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 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.
This can also happen in established tanks. The common cause for this is that something has impacted your filter bacteria. This could be improper cleaning or the dosing of a medication.
Something like Nitrico Goop, or one of the many liquid biological boosters available, will help cure this. Consider also adding more aeration in the tank, as the bacterial bloom will deplete oxygen levels.
2. Old tank syndrome
This can occur when a tank has been running for a long time (usually many years). Without adequate maintenance (especially adequate water changes) the bacteria can exhaust the tank’s supply of carbonates. This stops the bacteria from converting waste properly, and causes the water chemistry to fall out of balance.
The result is (amongst the things such as a pH crash) a sudden outbreak of white, milky cloudiness.
Re-establish the biological filter using something like Nitrico Goop, and address the underlying cause. The tank will need several partial water changes (around 25% a day over the course of a week). Perhaps even consider 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. If there’s more pollution than the filter bacteria can handle, other bacteria will grow to exploit it.
The solution here is clear. Address how much food the fish are receiving, and carry out a series of small water changes with fresh water.
4. Dead fish or plants
As with uneaten food, a dead plant or fish will produce a surge of pollution. Remove the plant or fish, and carry out series of water changes until you resolve the issue.
5. Natural decoration
Many aquarium decorations are natural in origin, and this organic matter can cause bacteria in the water. Some aquarium woods can cause water cloudiness soon after adding them to the tank. They may also have slimy-looking fungal growth over them.
Many botanicals such as leaves and seed pods can encourage a bloom of bacteria in the first few days after adding them. They will find a balance after a few weeks and they are nothing to worry about.
Algae related cloudy water
Algae is the blight of almost every aquarist. Usually it stays relatively manageable by clinging to glass and decoration. Occasionally it can be free floating in the water column and this can be a real menace.
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. This leads to a green tinge that can range from pale to pea soup coloured.
This problem can be because of excess nutrient being in the fish tank. As algae are single-celled organisms, they are typically too small for filters to remove.
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.
Controlling it can be difficult, waterchanges may only be a temporary fix. They can encourage green water to return stronger as new nutrients are introduce into the tank.
Adding chemicals like Interpet Green Away can help by flocculating the algae. This causes them to sink or make them large enough so filter can remove them. 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, 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 a 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 down to user error.
Common causes of chemical related cloudy water
1. Medication overdose
Many of the fish medicines we use kill 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 subsequent cloudiness.
Controlling cloudiness from a medicine overdose involves removing the excess medication from the water. Use a combination of water changes and the addition of activated carbon. The biological filter will need re-establishing 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 you add the powders directly to the tank, it’s quite common for them to create a whitish precipitate and cloud water.
Controlling this requires an immediate partial water change and gravel siphon. Mix any buffers prior to adding them to the tank in future and you won't experience it again.
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. 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 begin to form 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.
2. Loose connections
Typically, this affects tanks with external filtration (external canisters or sumps) mostly. As water passes at speed through pipework, if there are any tiny leaks, the rushing water can 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 checking every connection from tank to filter and back again. You should also check seal rings in canisters for damage.