Tips for dealing with cloudy aquarium water

If you’ve ever wondered why my fish tank is cloudy and what causes cloudy water in an aquarium, you need to read this guide! That dreaded cloudy water could be caused by bacteria proliferation during a new tank cycle, or there could be a more sinister cause. Whatever the reason for foggy water, you’ll need to know how to fix a cloudy fish tank.

Here’s everything you need to know!

What causes cloudy water in a fish tank?

First of all, what color is your water? It might seem like a strange question, but there are actually different shades of cloudy, and that tells you what’s causing the confusion, as well as giving you solutions to the problem.

white / grayish water

There are several causes of cloudy white or gray water in fish tanks.

gravel waste

If your aquarium water becomes cloudy with a grayish or white cloudiness by the time you fill a new tank, your problem is almost certainly caused by dust and dirt washing off the new substrate, usually gravel.

The solution for this problem is simple; simply empty your tank, remove the gravel and wash the substrate under running water until the water runs clear.

dissolved constituents

If your substrate is clean, but the water is still cloudy, the problem is most likely due to high levels of dissolved components, including heavy metals, silicates, or phosphates.

Test the tank water with a simple aquarium water test kit, and you’ll likely find that the water is very alkaline with a correspondingly high pH level. You can usually solve the problem by dosing the water with a tap water conditioner or pH buffer.

In some regions, tap water may be causing the problem. You can solve that problem by using reverse osmosis (RO) water instead of tap water. Most good fish stores sell reverse osmosis water, or you may prefer to purchase a reverse osmosis unit, which will be more cost effective if you have a large tank or more than one setup.

bacterial bloom

If you have a new tank and the water becomes cloudy within a few days or a week of installing it, the cloudiness is most likely caused by bacterial bloom.

Bacterial bloom, or bacterial bloom as the phenomenon is also known, occurs when a new biological filtration system is maturing. The problem usually resolves on its own within a couple of weeks or months. In the meantime, you can help the beneficial bacteria in the biofilter by vacuuming the tank thoroughly at least once a week to remove fish debris, uneaten food, and plant debris.

Sometimes reducing the amount of food you give your fish can help ease the load on the filter system, cleaning the water as a result.

Don’t turn off your filtration system

You can trigger bacteria growth by turning off your filtration system at night. Ideally, your filter should be running 24 hours a day, 7 days a week, 365 days a year, and you only need to turn it off briefly when you are maintaining the aquarium, or cleaning or changing the filter media.

How does flocculate help clarify cloudy water?

You can use water additives called floc to help clean the water. Flocculate works by causing small particles of floating debris to clump together so that the mechanical element of your filtration system can remove them from the water, effectively curing cloudiness.

You’ll find flocculates for sale at your local fish store, labeled «water clarifiers.»

Watch out for big water changes!

Many hobbyists inadvertently cause bacterial overgrowth after a large water change that upsets the delicate ecological balance within the aquarium. Removing too much water from your tank can start a mini-cycle, when the colonies of beneficial bacteria that live in your tank and on the filter media begin to repopulate.

So limit yourself to partial water changes of around 20% to 25% each week, and if you need to change half the water in your tank for whatever reason, be sure to add a filter booster product to the water.

Green water

Cloudy green water in your fish tank is most likely caused by algae growth in the water.

Algae are tiny aquatic organisms that are often found growing everywhere in your fish tank, including on the substrate, glass, decorations, and even on your plants. There are also some species of algae that live in the water, floating around and giving the water a cloudy, greenish tint.

How to get rid of algae in aquarium water

Getting rid of algae in your fish tank is not easy once it gets established. Since algae can be caused by a number of things, you should use a multi-pronged attack strategy:

Reduce excess light

Too much bright light, especially sunlight, can encourage algae growth in the tank water. You can restrict algae growth by moving your tank away from sunlight or by protecting the aquarium with blinds or curtains for the part of the day when the sun shines on the tank. Also, you should reduce the hours that your aquarium lights are on, perhaps by using a timer.

Once the algae lack light, they will eventually die, and your filter system will remove the green algae from the water, making it clean again.


Phosphates that are derived from the decomposition of waste organic matter in the water and from the water source itself are effectively plant fertilizers that encourage algae growth.

To reduce the amount of phosphates in the water, start by thoroughly cleaning the tank, which includes trimming dead plant matter and vacuuming the substrate to remove fish waste and uneaten food.

Once this is done, try the tap water as that could be the source of the problem. The Safe Drinking Water Act (SDWA) of 1974 mandates that the US Environmental Protection Agency is required to chemically treat domestic water supplies to make them safe to use and drink. That process means the water is chemically treated, often resulting in high phosphate levels.

If your domestic water supply contains high levels of phosphates, you will need to switch to using RO water for your aquarium or chemically treat the water to remove the phosphates before putting the water in your tank.


Nitrates also act as fertilizers for plants and algae.

Nitrate levels in your fish tank generally increase in direct correlation to the amount of fish waste in the water, and ideally, your aquarium water should contain no more than 20 ppm (parts per million) of nitrates. As long as you do partial water changes of up to 30% each week and maintain your filters properly, nitrate levels should be fairly easy to control.

In order for the biological element in your filtration system to work efficiently and remove nitrates from the water, you must have a good flow rate throughout the tank. Ideally, you want a GPH (gallons per hour) flow rate of at least four times the total volume of water in your tank, preferably more.

yellow/brownish water

Although it is quite unusual for the water in the tank to turn a dark brown or yellowish color, it can sometimes happen.


Many hobbyists use bogwood and driftwood as aquarium decorations. Wood looks very natural, some plant species can be trained to grow on wood, fish use wood to determine territories and use it as hiding places, and colonies of beneficial microbes grow on wood surfaces, providing food for fish and fry.

However, the tannins contained in the wood can seep into the water, slightly lowering the pH level and sometimes turning the water the color of iced tea. Some fish species appreciate a «black water» environment because it mimics their natural habitat, but cloudy yellow or brown water doesn’t look very good and makes the tank look dirty, which it really isn’t.

How to prevent «black water» in your fish tank

You can prevent «black water» from forming in your tank by soaking driftwood or bog in a bucket for a few days before placing the wood in your aquarium. Of course, any tank decorations you buy at a pet store should be prepared, so you don’t have that problem. If you have an activated carbon filter, that will also help remove some of the discoloration.

Dry leaves

A layer of dry leaves on top of the substrate can help replicate the natural environment of some fish species, giving the tank an Amazonian feel and providing a place for colonies of microbes to grow that can provide food for fish and fry.

Dried leaves also leach tannins that will give the water a brown color. Unless you want to create a «black water» habitat, you will need to remove the leaves to allow the water to clear again.

How to prevent cloudy fish tank water

Avoiding cloudy fish tank water is quite simple. In addition to the fixes mentioned above, there are a few rules of thumb that you will need to follow to keep your aquarium crystal clear.

Avoid overfeeding

Overfeeding your fish is bad for their health and will also make your aquarium water cloudy due to a proliferation of bacteria or uneaten fish food particles floating in the water.

I recommend that you feed your fish only what they will eat in a couple of minutes. Feed fish little and often, especially species like bettas that can become constipated if overfed. It is also beneficial to have one day a week where you do not feed your fish at all. That’s not cruel; Fasting helps keep the fish’s digestive system healthy and also prevents associated health problems.

Avoid overcrowding

Overstocking your fish tank is causing problems on multiple levels. Too many fish crammed into a small space can cause territorial species to become very aggressive, and a lack of personal space also causes stress, which can also result in health problems and disease outbreaks.

Overcrowding an aquarium with too many fish will overload the filtration system, overwhelm the beneficial bacteria that process waste products, and lead to bacterial overgrowth and cloudy water. You can solve the problem by relocating some of your fish to another tank, doing a partial water change, and vacuuming the substrate thoroughly.

Add activated carbon medium to the filter

As mentioned above, adding activated carbon to your filter media can help remove tannins that can stain your water a brownish-yellow color.

seed the aquarium

If you have a new setup, you can prevent bacteria growth and cloudy water by seeding the aquarium.

Stocking involves transferring some substrate, plants, or decorations from a mature tank to a new one. With those items come the beneficial nitrifying bacteria that are essential to starting the tank cycling process. Seeding can cut the usual cycle time in half, as well as reduce the chance of cloudy water.

In conclusion

So if your aquarium water becomes cloudy, you’ll need to do some detective work to determine the cause of the problem.

New tanks can become cloudy because they have not completed the cycle before the introduction of the fish, which causes bacteria to proliferate. If the tank is overlit for too long or exposed to direct sunlight, floating algae can grow, turning the water green. Finally, if you keep bogwood, driftwood, or dried leaves in your tank to give it a natural look, you can inadvertently create a blackwater habitat where the water turns a yellowish-brown color due to tannins that have seeped into the tank. Water.

We hope you enjoyed our guide on cloudy aquarium water, but if you have any questions, be sure to put them in the comments section below!

Check out our infographic and don’t forget to share it!

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