5 Reasons Why Guppy Fish Tank is Cloudy

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Having a thriving fish tank will involve knowledge and some effort on your part to make sure that the fish population remains stable and safe. But, sometimes, things can take unexpected turns that will require your intervention.

Such a situation is the tank water turning cloudy and murky. What should you do then, what are the causes, and how can you prevent that?

Let’s look at the 5 reasons why fish tank water usually gets cloudy.

1. No Nitrogen Cycle

Simply put, the nitrogen cycle is a natural phenomenon that occurs in all ecosystems, including manmade ones like the aquarium. The nitrogen is a chemical that circulates between the various mediums as part of a biological chain reaction. The cycle begins with fish waste and decaying food-producing ammonia in the water.

Ammonia is consumed by the nitrifying bacteria that occur naturally in the aquatic environment, neutralizing the harmful chemicals and transforming them into nitrites. These will also serve as food for other different strains of nitrifying microorganisms, which will convert nitrites into nitrates.

The problem occurs when the ammonia buildup happens at a faster rate than the nitrifying bacteria can counter it. This will create a toxic environment for fish, leading to ammonia poisoning, which is deadly.

2. Overfeeding

Overfeeding is a common problem that many guppy owners will face. Many people think that the fish’s appetite is an indicator of its real nutritional needs, but that’s not the case. Guppies will happily eat more than they should since this is their biological programming, like most other creatures.

In reality, guppies don’t eat as often in the wild due to scarce resources and high food competition. Your guppies should eat twice per day at most and only what they can consume in 1 minute.

This is especially true if you’re feeding guppies more protein and fats than necessary. Overfed guppies will gain weight, experience constipation, and deal with a variety of digestive issues. 

But the main problem linked to overfeeding is the chemical imbalance it causes in the water. Unconsumed food will accumulate on the substrate, decay, and increase ammonia levels, leading to dangerous nitrite buildup.

The situation is at its worst when the water becomes cloudy with free-flowing particles flooding the tank. This is a nightmare scenario that should never happen.

3. Dirty Filter

The purpose of a tank filter is to clean the water, remove deadly ammonia, control the nitrate levels, and oxygenate the environment so your fish can breathe. It’s an essential addition to your tank, especially if you have multiple fish.

The problem occurs when the filter clogs or becomes so dirty that it can no longer cleanse the water. The first sign is the accumulation of particles, making the water seem cloudy and murky. At this point, cleaning the filter is a must, but with care and attention.

You don’t want to overdo it or clean it with chemicals that will sterilize it. The tank filter harbors a variety of beneficial bacterial cultures that contribute to the tank’s ecosystem. These are microorganisms that consume ammonia and nitrites, keeping the water clean and safe for your fish.

Sterilizing the filter will kill off the beneficial bacteria, disrupting the tank’s sensitive ecosystem. Paradoxically, your excessive cleaning habits can do more harm than good.

You should always clean the filter with care to unclog it and remove the dirt without destroying the beneficial bacterial cultures. And refrain from using cleansing chemicals that could sterilize the device.

4. Disturbed Substrate

This is another common cause for cloudy tank water, generally occurring in tanks with bottom-dwelling fish species. These fish will often look for food in the substrate and may cause a mess in the process. If this happens often enough, the tank water may become constantly cloudy and messy, making it unfit for your guppy population.

The same problem arises due to human intervention, with guppy owners trying to clean the substrate and causing more problems than they fix. 

I recommend using a specialized substrate vacuum to clean your tank with minimal fallout.

Also, try not to have too many bottom-dwelling creatures that could disturb the substrate. These can affect water clarity and cleanliness, affecting the entire guppy population in the process.

5. Huge Water Change

This may appear counter-intuitive, but it actually makes sense. Let’s consider how the fish’s environmental parameters change over time. Guppies are hardy and adaptable fish, and they need to be since their natural environment changes constantly. While their habitat’s parameters don’t fluctuate wildly, they will fluctuate, and guppies have learned to adapt to these changes.

The same happens in a tank. You may change water weekly, but many people don’t. Some guppy owners change the water once every several months or even years. This means that the guppies have already adapted to their changing environment since not changing the water won’t kill your fish. At least not immediately.

Guppies have managed to adapt to a dirtier environment because the changes were incremental. They won’t show the same adaptability in environments where changes occur rapidly or, even worse, abruptly.

This is the case with massive water changes, suddenly disrupting guppies’ environment without giving the fish time to adapt. Paradoxically, massive water changes meant to provide guppies with a safer and cleaner environment can kill them in the process. The risk is even higher when you haven’t performed water changes for a long time.

But what does ‘massive water change’ mean?

Ideally, you should change 10 to 20% of the tank water weekly to provide your guppies with a stable biosystem. Sticking to this ratio is essential to clean the water without disturbing the fish too much.

A 50 to 70% water change is out of the question. This is a too dramatic environmental shift that could kill your guppies as a result.

You want to go for weekly, steady, and controlled water changes with minimal discomfort for your guppies.

How to Fix Cloudy Fish Tank?

First, you need to identify the cause. There may be multiple causes for cloudy tank water in many cases, each requiring a different approach. Depending on the causes, here are some potential solutions you can apply:

  • Ensure a healthy fish-to-gallon ratio – When it comes to guppies, you should have at least 2 gallons of water per fish. You may need more in some cases, depending on how big the fish is or whether you have a lot of tank decorations and plants taking up space.
  • Clean the tank regularly – Vacuum the substrate and remove dead matter, fish waste, rotting plants, and food residues. This alone will prevent waste accumulation, keeping the tank water cleaner for longer. It will also help you to counter algae growth which could affect the water’s cleanliness.
  • Avoid overfeeding – Don’t overfeed your guppies. Feed them once or twice per day at most and only what they can consume within 1 minute. This will minimize the food residues that will decrease the water quality and promote dangerous ammonia buildup.
  • Have a reliable filtering system – Guppies don’t always need a filtering system, but I recommend having one anyway. The investment will be worth it long-term.


If your tank water is cloudy, the problems have already advanced too far. Identify the cause fast and start making changes today, before your guppies start to fall sick and die. Unclean waters may not kill your guppies right away, but they will decrease their immune systems, leaving them vulnerable to parasites, bacteria, and disease.

Ensure regular water changes, clean the tank occasionally or whenever necessary, and monitor water parameters constantly. The latter will allow you to prevent any problems with the water quality that could turn deadly if left unchecked.

Updated: December 10, 2021
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