Cloudy Fish Tank Water – Main Causes & How to Fix it

Updated on by Patrice Alsteen | Affiliate links may be present.

Cloudy water in a fish tank is often a signal that something’s gone awry and immediate action is in order. The water could turn white to gray, green, or yellow to brown.

There is no single or simple answer to why aquarium water changes color. There are many possible causes and potential problems that can cause this. However, there are characteristic sets of problems for each color.

Let’s dive deeper and explore the potential causes and solutions to cloudy fish tank water.

White to Gray Water

When the water gets hazy or clouded in a fish tank, it can often turn white or grayish. There are several things that can cause this to happen. Luckily, there are fixes for each. Let’s investigate.

Gravel Problems

One of the most common reasons behind fish tank water turning white to gray is gravel residue. This happens most commonly within the first couple of hours after you have filled the tank with fresh water. The gravel was probably not washed and rinsed well enough.

To remedy this, you should empty your tank and rinse the gravel thoroughly under the tap. Don’t stop until the water is clear. Fill the tank once more and put the gravel back in.

Dissolved Chemicals

A high content of dissolved chemicals can cause the water in your tank to go white or gray. The most problematic chemicals include silicates, phosphates, and heavy metals, though other chemicals may be at fault too.

If a chemical is to blame, chances are that the water is alkaline (high pH).

In order to solve this situation, you might want to introduce a conditioner that will reduce the pH in the tank. Another potentially better solution is to use reverse osmosis (often referred to as RO) water.

Blooming of Bacteria

If it took the water in your tanks several days, weeks, or months to turn white or gray, you can rule out gravel problems and dissolved chemicals. The most likely culprit in this situation is bacterial growth.

Most of the time, bacterial blooms are nothing to worry about and they tend to go away on their own. Properly formed bacteria colonies tend to take care of occasional cloudiness. You should keep the tank clean, remove uneaten fish food, take out dead plants and fish, clean the gravel often, and you’ll have very few problems with bacteria blooms.

If you can’t remove the debris through conventional ways, you should introduce flocculates to your aquarium. They cause bits of debris to cluster together, and your filter will be able to get rid of them more easily.

You might also want to consider switching to feeding every other day or even every third day to minimize rotting food debris. Finally, make sure the water filter’s clean at any given time.

Green Water

If you see green water in your fish tank, there’s only one reason for it – algae growth. However, banishing it is a completely different story. Here are the main causes of algae growth and how to battle it.

Excessive Light

Excessive light is to blame in most cases. Accordingly, it is the easiest to solve. If your aquarium is receiving too much light, it will develop algae.

To remedy this, you should move the tank away from direct sunlight if it’s exposed to the sun. If not, you should reduce the amount of time the lamp is on. Once the amount of light’s been brought to an optimal level, the algae should disappear and the water in your tank should return to its natural state.

Piled-Up Nitrates

All fish start releasing nitrates into the tank the moment they’re put in; that’s their natural way of getting rid of waste. Therefore, it is only natural to expect it to rise if tank maintenance is poor. Also, the more fish there is in the aquarium, the faster the nitrate levels will grow. Piled-up nitrates can damage fish gills and become lethal if left unchecked for a long time.

To remedy this, you should clean the water regularly. Also, make sure there’s an appropriate amount of water for the tank size. Finally, you should keep the number of fish at the right level for your tank.

You can opt to replace the water, but if you don’t deal with the underlying cause of heightened nitrate levels, this problem will come back.

Piled-Up Phosphates

Similar to nitrates, it is very important to keep the level of phosphates in your aquarium in check. If the levels get out of control, phosphates can promote the growth of algae, which will, in turn, color the water in the tank green.

You can change the water but this alone will only provide temporary relief. It is crucial to reduce the levels of phosphates as soon as possible.

The best way to determine the concentration of phosphates is to test the water. If the levels are too high, your best bet is to employ RO water in filling up your tank. Almost all local fish stores stock RO water or has the facility to make it.

Alternatively, you can get a phosphate remover and put it in the tank. As an added measure, you might want to consider giving your fish less food. Also, you should switch to a brand that contains lower levels of phosphates.

This study published in 2013 examined the aquatic toxicity of dicalcium and tricalcium phosphate. While it found no conclusive evidence of their toxicity, it determined that they promote algae growth.

Yellow to Brown Water

In all the previously mentioned cases, cloudy aquarium water is caused by human error. However, sometimes the water can turn yellow or even brown. Though it might be alarming to see your tank take on the color of tea, it is not through your mistake and it is not harmful.

The water can turn yellow or brown because of driftwood. This mostly happens with newer pieces of driftwood, though older pieces can cause the problem as well. Newer driftwood pieces tend to make the water darker. This happens because of tannin, a dye that’s used to color driftwood.

Driftwood is actually beneficial to fish and many fish owners add it to their tanks. For one, it can lower the pH levels, thus creating a healthier environment for the fish.

If you add a piece of driftwood to your tank and the water starts turning yellow or brown, take the piece out and rinse it thoroughly. You can even boil it to get rid of all tannin. For any tannin-tainted water, you can add a carbon filter to get rid of it.


Cloudy water in a fish tank is by no means a pleasant thing to see. Most often, though, as is the case with white and green water, it is a sign of some underlying maintenance problem. Yellow is the only benign color, as it is caused by driftwood dye.

About Patrice Alsteen

Patrice has been an animal lover, right from her childhood. She's currently enjoying the company of her cat, dog, aquarium fishes, and a few small furry companions.