What causes cloudy water in a fish tank and how to fix it

What causes cloudy water in a fish tank

What causes cloudy water in a fish tank? Well, that’s a query beginner aquarists find answers to from first hand experience.

Like a ritual, you’ll experience cloudy aquarium water at some point. No two ways about it. Longtime aquarists can attest to this.

However, even if you are a beginner, it’s nothing to worry about. That’s if you have the right information though. In the meantime, just know that it’s not without reason that aquarium’s waters experience drastic changes.

The good thing is that, it’s not always an emergency case. In-fact, the color of the water helps you to predict the cause of the haziness. Besides, cloudy water in fish tank solutions abound for each cause.

Most importantly, be able to decide on the right course of action or inaction (funny that they both are possible solutions).

What causes cloudy water in a fish tank? Using colors to determine that:

Firstly, it’s safe to know that this will occur in both new and established tanks for various reasons. The colors to watch out for include;

Green color;

This is as a result of algae bloom, usually suspended in water. Unfortunately, it’s a form of algae that critters and algae eating fish won’t eat.  While it’s not much of a threat, it’s worth finding out the actual cause and factors supporting the phenomenon.

The causes narrow down to some of the factors below. They could be working singularly or take any combination.


If you asking yourself, “why is my fish tank cloudy after water change?” Well, it most probably is because of phosphates in water.

However, a phosphate test is first recommended. In addition, it should be both on the tank waters and water source. This eliminates speculation. Moreover, it will help you focus your counter measures.

In most cases, phosphates come about through the water added to the aquarium.

Water sources are a major way of getting phosphates into the aquarium. You ask how? Well, for instance, the fertilizers used in the farms are a major source of phosphates in water. Naturally, the unused portions of it by plants percolate into the soil. Eventually, they are carried into underground water reservoirs.

It’s from there where they are introduced into the tank should that be the water used. Subsequently, they fertilize the algae bloom causing rapid increase.

Further, excess fish excretes mostly in cases of overstocking, and decay of uneaten foods and dead matters say of fish and plants is another source of phosphates in the tank.

Phosphate is also be introduced into the tank through fish foods which have it as a preservative. That’s however though not in large proportions and also not too common.

Likewise, it’s also present in some chemical varieties for pH adjustment additives.


These practical suggestions below will help in your “how to fix cloudy water problem” caused by phosphates in water.

  • Reduce overfeeding besides doing frequent water changes.
  • Further, a phosphate absorbing filter media will help you in cleaning off the phosphates in the tank.
  • Sort the water source issue. That, you can effectively do with a reverse osmosis (RO) filtered water instead of using your normal tap/borehole water.
  • You can do with the help of flocculates, sold as water purifiers. They roll-up together the uneaten food particles and debris into small ball-like particles for easy filter removal.
  • Having healthy aquarium plants. While they are not the silver bullet for a cloudy aquarium water problem, they are helpful in “eating” away harmful compounds. Compounds like ammonia and iron are greatly active in spurring growth of algae and bacteria. 


Nitrates are purely a by-product of waste (both poop and pee) from the fish. They are naturally occurring overtime.

The breakdown of this waste is what results to the toxic ammonia. Ammonia, besides burning the gills of your pet fish is toxic to the fish’s lungs. Unchecked, you fish will agonizingly die in days.

However, fish waste is processed by the beneficial autotrophic/good bacteria (read about them here)   that colonize the filters. This essentially means that they are food to organic substances. Some effectively break ammonia to nitrite and others make the less toxic nitrate from nitrite.

The tank’s life is safe if under 20ppm nitrates levels are maintained. Meanwhile, even 1ppm of either/both nitrite and ammonia is enough to kill your pet fish.


You just need to have a clean tank by

  • Doing nitrate levels tests both before and after water changes,
  • Regular water change,
  • Have sufficiently and functionally effective water filter(s) with nitrate absorbing media. To remove the dissolved nitrates, an anaerobic denitrifying bio-filter will be just fine.
  • Moderate stocking to reduce levels of waste in the aquarium.

Excess Light:

Among what causes cloudy water in a fish tank is light. Yes, light!!! May sound misplaced but true.

Like with all other plants, light encourages the growth of Phytoplankton, the floating algae bloom on the aquarium’s surface.

Light and nitrogen are the best enablers of algae bloom. This subsequently causes a greenish foggy aquarium.


Unlike the other causes, this is pretty manageable. Why? Because it’s not a result of biological functions. Those, you don’t have control over.

In this case, how to get rid of cloudy water is by reducing exposure to too much light. Subsequently, you’ll be underfeeding the algae. This you can do by ensuring;

  • Keep the tank away from direct sunlight,
  • For the indoor lighting, ensure the lights are not on unnecessarily long,
  • Using algae scrappers, just scrub off the algae,
  • Use a fine filter media or the UV sterilizer,
  • If your light bulbs are overly old, they lose their lighting spectrum (basically the color), hence exposing your tank. How? You ask. Well, while the poor lighting color by old bulbs will be feeding the algae, it’s utterly unhelpful to the good aquatic plants.

Grey/White color;

Your aquarium waters could turn milky or thick white fog because of a few reasons which may not necessarily be too worrisome. However, I will educate you on all those that are a concern which also depends on when it happens.

The Bacterial bloom:

This happens in different scenarios and instances. They include:

  • A new aquarium set-up:

Is your new tank cloudy? Well, don’t worry.

Bacteria bloom in fish tank is prevalent during setting up of aquariums. However, the milky water spectacle doesn’t happen immediately after. Well, not even in 3-5 days but it’s definitely inevitable.  

For a new tank, this is simply referred to as tank cycling process. Subsequently, new tank syndrome cloudy water results as the beneficial bacteria colonies are building up.

Why is that buildup important? Well, the decaying of the fishes’ waste and feeds produces ammonia. This is a toxic compound harmful to the fish. It is therefore the work of these beneficial bacteria to neutralize the risk of ammonia; by consuming and breaking it down. 

Consequently, the multiplication of these bacteria floating in the water results in the milky water.

Key point is that, when the cloudy aquarium water appears, test the ammonia and nitrite levels. They MUST be at NIL ppm (0 parts per million).

Now, you could be wondering; will cloudy water kill my fish and just how long does it take for cloudy aquarium water to clear? Well, the floating bacteria will take 2-3 days to clear off. Further, because it’s short-lived, it will not harm your pet fish.


The rapid bacteria buildup may cause an oxygen debt/shortage in the tank which isn’t good for the fish. However, that shouldn’t take away your sleep. Dropping an air pump in the tank or doing 20%-30% tank water change to dilute the waters will alleviate the problem.

  • During an aquarium restart:

Any experienced aquarist will tell you that a cloudy fish tank water in established tank is a common occurrence.

Mostly, it results during a tank’s restart. Well, a restart is just a rebuilding back of the beneficial bacteria colony again in an old/established tank.

For instance, medications say antibiotics will kill the bacteria, or introduction of fresh water from untreated sources. As a result, the levels of the bacteria reduces or are eliminated altogether.

This will essentially lead to a need for fresh rebuilding. Subsequently, when this is happening, it will result in a foggy aquarium.


In this case, it’s unlike when it happens in a new tank set-up where “doing nothing” is recommended. In the meanwhile, the biological functions are naturally settling the issue. So, rescue measures here include;

  • Use water conditioners if tap/chlorinated water is used in a water change,
  • Because water filters holds large colonies of these bacteria, it’s recommended you clean them with water from the aquarium. Why? Because it prevents washing them off. This will necessitate a new cycle.

Overstocking the aquarium: cloudy water in established tank;

So, why is my aquarium water cloudy on account of overstocking? I guess it must surely be sounding out of place.

Well, two things happen. Firstly, this will lead to increase in waste in the tank. Secondly, there is the obvious extra feeding which results in uneaten feeds in the tank.

The waste and the decay on the feed remains cause a spike of ammonia in the tank. Subsequently, this result in triggering rapid beneficial bacteria buildup to counter the ammonia and the nitrite that arises out of ammonia breakdown.

The result is aquarium water turns cloudy because of good bacteria colony building.

  • Filter or filter medium replacement:

Remember the recommendation above on washing off filters with tank waters? Just any water cleans off the beneficial bacteria colony on the filters.

Likewise, if a new filter is being introduced, hazzy aquarium water as a result of bacteria bloom will result too.

This is as a result of the bacteria doing a colony rebuilding.

Similarly, this will happen if you restart filtration after having been non-functional for a considerable duration. Prolonged lack of water movements depletes oxygen in the tank. Subsequently, this kills the beneficial bacteria.


  • To retain part of the built-up bacteria colony in the new filter, ensure at-least half of the filter medium is from the established filter,
  • Avoid prolonged filtration breaks because they also cause grey cloudy aquarium water.

Aquarium gravel residues: From improper cleaning;

This can happen in both new and established aquariums;

  • In-case of new substrate:

Well, a lot of shuffling and rough tossing around of substrate bags happens; on the way to the stores from manufacturers or even from the store to your residence. As a result, dust like particles will form from the rubbing and brushing off of substrate in the bags.

Now, before introducing it into the tank, improper cleaning is essential. However, if not done thoroughly, the dust remnants will subsequently result in hazy cloud in the water.


  • To trap these very fine particles, you can use filters with super fine mechanical filter media,
  • You could also do a 30%-50% water change. This will dilute and reduce dust levels in the water.
  • In-case of an established tank:

The cloudy water may still persist even after proper tank vacuuming and gravel washing. This could point to un-dissolved elements like heavy metals and silicates among others. This you can sort with water changes as well as using water conditioners available in pet stores near you.

Brownish/Yellowish color: Why cloudy aquarium water?

This phenomenon is characterized by a brown cum yellow coloration in your tank. Gradually, if not stopped while still at a yellow tinge stage, it eventually will be dark like sweet colored tea.

Tannin (dissolved organic compounds) that leach in your tank waters is what causes cloudy water in a fish tank on introduction of driftwood.

It’s basically the “brownish” dye like color from soaked leaves and woods that leach into the aquarium.

While driftwood, which fish love rasping and nibbling on, is the common source of tannin, some water sources have it too.

Tannins make plants bitter and distasteful as a defense against insects. However, tannin is harmless to both your fish and aquarium plants.

Just a note though, as a personal preference, some aquarists like their aquariums yellowish/brownish. Resultant aquariums are commonly referred to as black water aquariums. To do that, untreated driftwood and leaves are added into the tank.

Some of the known effects of driftwood’s tannin is water pH lowering and softening hard water. Additionally, besides being known to improve immunity of the fish, it also adds sparkle to their color.

An even greater bonus is that, it stimulates spawning as well as improving their appetite.

How to clear cloudy aquarium water and remove the brown color?

The driftwood type determines the amount of tannins deposited into the tank. Some have a mild shade than others.

However, pre-soaking (even better if boiled to hasten) removes the tannins before placing the wood in the aquarium. Fortunately however, besides regular water changes, say weekly (in moderation), a filter with activated carbon will also do a good job in removing the coloration already in the tank.

Large driftwood decoration pieces may still leach tannin even after a considerably lengthy soaking period. Well, you will witness small color change effects over time. 

Caution however is to NOT soak in containers with either chemicals or detergents traces.

Secondly, use purified water only.

How to clear fish tank water fast:

After knowing what causes cloudy water in a fish tank, how to fix it should be easy. Here are three recommendations to start with.

However, as long as the hazy water problem persists, you must explore further options to salvage the situation.  Luckily, almost all corrective measures available have been discussed here above for every cause of cloudy aquarium water.

  1. Gravel and substrate vacuuming regardless of what is causing cloudy aquarium water. Ensure to remove as much waste and all matter as possible for the obvious reasons.
  2. Well controlled water tank changes. Ensure to do between 15%-30% tank water change during every change. It MUST NEVER be 100% at any one change.
  3. Controlled stocking and overfeeding avoidance.
  4. A reliable clean-up crew: These are fish and other invertebrates that clean up your tank by consuming waste and algae in the aquarium. This prevents decaying effects in the tank because harmful compounds will not build-up.

These include but not limited to Otto cats, Siamese algae eaters, amano shrimps among others.


I trust that you have now appreciated that cloudy aquarium water is like a rite of passage for every hobbyist.  Further, whenever it happens, it definitely will show in any of color shades discussed above. That’s a good thing because it will be easier to do a quick diagnosis.

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