Fish’s Mouth Moving Rapidly: Reasons & Solutions

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As a fish owner, one of my favorite things is to observe my tank. There is no doubt that something is relaxing about it. However, a few times, I noticed that my fish tend to open and close their mouths quite rapidly. That phenomenon got me worried, so I started researching the topic a little deeper.

Fish tend to open and close their mouths rapidly when they experience low oxygen concentrations. By consistently gulping the water, the chocked fish create a flow over their gills. That allows them to take advantage of the limited dissolved oxygen presented in the water.

As we move forward in this article, I will show you which cases could cause this phenomenon, and what steps you should take to avoid them. That will allow you to prolong the lifespan of your fish, and possibly save them from a cruel faith. 

Rapid Mouth Movements in Fish: Why is This Happening?

Fish don’t breathe like humans; they have mouths that can take water in. However, they rely on their gills to meet the need their bodies have for oxygen. When water passes over the gills, the oxygen is extracted and absorbed into the blood. Carbon dioxide, on the other side, is eventually expelled. 

For fish to breathe freely, water must consistently flow over their gills.[1] This is why they gulp water every so often. This helps the process. Put simply; you have no reason to worry just because your fish keeps opening and closing its mouth.

Naturally, this can become a source of concern for some fish owners. Because they know that fish breathe through their gills, they do not expect the creatures to use their mouths when not eating. But this reasoning is wrong. 

You should only worry if your fish is opening and closing its mouth frequently and rapidly. This sort of mouth movement typically means that your fish is gasping for breath. The reasons for the phenomenon tend to vary. They include:

1. Elevated Ammonia Concentrations

Ammonia can be toxic, and high concentrations of it are sometimes blamed on overfeeding.[2] That is since ammonia forms from the waste that your fish excretes. It can also emanate from leftovers in the tank that are allowed to rot. Both factors have a direct connection to overfeeding.

On high levels, ammonia is likely to burn the gills of your fish. Because the creatures use their gills for breathing, their breathing will become labored, leading to rapid mouth movements. 

One way of identifying troubling ammonia concentrations is to look for lethargy in your fish. If you’ve noticed that they start swimming slowly and spend a lot of time at the bottom, that could indicate an issue.

Another indicator is the lack of appetite. If your fish show no interest in food, that typically means they suffer. This could create a vicious cycle since leftovers will raise the concentration of ammonia even more. 

2. Toxic Nitrites

Ammonia is not the only toxic element you have to worry about; nitrite is just as problematic (despite being less toxic). It binds to the cells that are responsible for carrying oxygen. By inhibiting their operations, it causes the fish to suffocate. As a result, your fish will gasp for air, moving their mouths rapidly. 

You may also notice that your fish are rising to the surface. Some people presume that fish only gasp for air at the surface when the oxygen levels in the tank have fallen. But nitrite poisoning can achieve a similar effect.

3. Low Oxygen Levels

One of the most common causes of labored breathing and rapid mouth movement in fish is oxygen deficiency. The reasons for that in a fish tank tend to vary. For instance, high temperatures can reduce the water’s ability to hold oxygen.

This is why domestic pet fish sometimes struggle to breathe in the summer. Also, a malfunctioning heater can produce similar results by overheating the water. This is why you are encouraged to watch for signs of erratic temperatures.

If your aquarium water is too warm, the gasping you’ve noticed among your fish is going to evolve into a far bigger problem down the line. Continues hypoxic states could lead to diseases, and possibly death. 

4. Carbon Dioxide

Fish naturally take in oxygen and, in return, expel carbon dioxide. If the concentration of CO2 is allowed to grow to unsafe levels, your fish will start breathing rapidly, gasping for air. If the rapid mouth movements of your fish are accompanied by erratic swimming, dangerous carbon dioxide levels could be the cause.

That could be the case in relatively small tanks that are overpopulated. The carbon dioxide will be secreted faster than oxygen is being dissolved. An aquarium filter could partially solve that issue, although in many cases, it isn’t enough. 

5. Diseases

Some diseases and infections can affect the gills, inhibiting your fish’s ability to breathe. That may quickly lead to air gasping. If your fish has become the target of an infection that attacks the gills, you should speak to a professional.[3]

You can always tell that your fish is sick by looking for spots, clamped fins, lethargic swimming, and the like. But diagnosing gill-related diseases could be challenging. That is why you should speak to a specialist before your fish deteriorates. 

6. Hydrogen Sulfide

Hydrogen sulfide can form if you fail to maintain your tank appropriately. It is usually identified by a smell similar to rotting eggs. That, in turn, can suffocate your fish by making sulfide using the iron in their blood cells. The next thing you’ll notice is rapid mouths movement due to suffocation.

It has been reported that moving large ornaments in the aquarium could lead to that issue.[4] When placed in the same spot for an extended period, the gas gradually accumulated underneath these objects. Untrapping it could elevate Hydrogen Sulfide levels pretty quickly.

How to Stop Your Fish from Moving their Mouths?

If you want to stop your fish from rapidly opening and closing their mouths, you have to find and resolve the cause of their behavior. This typically involves the following:

1. Ensure the Temperature is Adequate

The first thing you should do is getting a high-quality heater. Do not rely on the ambient temperature to control the conditions in your tank. Buy a heater that is powerful enough to regulate the temperature in your tank. 

If you suspect your current heater doesn’t deliver, I highly suggest that you read my aquarium heater buyer’s guide. In there, I reviewed the device I personally use (with great success). I also attached a graph that illustrates how stable the water temperature in my aquarium actually is.

Then, make sure to measure the temperature regularly to ensure that it is within the appropriate range for your particular fish. This will allow you to catch any heater malfunctions before they wreak havoc on your pets.

2. Regulate Toxins

Like the temperature, you should take regular measurements to ensure that the ammonia and nitrite levels are at zero. You should also keep an eye on the carbon dioxide levels, not to mention the hydrogen sulfite, which forms in the gravel and filter.

Also, make sure that you clean the bottom of your tank regularly. The same goes for the filter, especially when you experience power outages. If you restart a filter without cleaning it after a power outage, all the accumulated toxins will enter the tank.

Carry out regular water changes. This is the only way to control the nitrite and ammonia concentrations. Don’t wait for problems to arise to do this. Though, if your fish are already gasping for breath, making a partial water change should be among your first steps. 

Lastly, do not forget to check the biological filter to ensure that it is working as expected. If necessary, disassemble it entirely and wash each part under running water. Once all the debris got washed off, put it back in and see if there is any improvement.

3. Control the Number of Your Fish

It is challenging to maintain an overcrowded tank. The more fish you have, the more waste they will produce. Over-populated tanks are prone to oxygen deficiencies. Hence, you should act to reduce the number of fish in your tank. 

Fish stores are typically quite happy to take fish back if you no longer want them. That could also be the faith of the offsprings of your fish. Even though it is pleasant to keep different generations of fish, eventually, you’ll have to give some of them away.

4. Take Advantage of Air Stones and Plants

One way of raising the oxygen levels in your tank is to add powerheads or air stones to increase the surface water movement.[5] This will improve the distribution of oxygen throughout the aquarium significantly. 

Your other option is to introduce plants, which will absorb CO2 and generate oxygen.[6] If your tank features low levels of oxygen and excessive carbon dioxide, add some vegetation. Of course, you’ll have to use real, living plants. Plastic ones cannot offer this benefit. 

Also, do not forget to watch for signs of disease. If you notice a lack of appetite, bloating, an accumulation of mucus and discolored gills, consult professionals. They will guide you on the best way to diagnose and treat your fish.

Is it Normal For Fish to Open and Close their Mouth?

It was mentioned above that fish can use their mouths to take in water as a means of enhancing the work their gills do. But the frequency with which this happens is quite low. For the most part, unless your fish is eating or yawning or even fighting, its mouth should remain closed.

If your fish is opening and closing its mouth, it should happen slowly. That indicates normal behavior. However, if the mouth movements are rapid, then something has gone wrong, and your fish is probably struggling to breathe.

In the last case, you should take steps to identify the source of the oxygen deficiency. Then, as was mentioned earlier, you can take the steps needed to solve the issue. You may measure, clean, and oxygenate your water accordingly.

How do You Know if Your Fish is Dying?

Fish cannot speak. Therefore, you cannot rely on them to tell you when they are sick or, even worse, dying. However, there are signs that you can watch for. They include:[7]

  • Try taking your fish out of the tank – Once your net starts scooping it from the water, a healthy fish will struggle to escape. If your fish is just lying there and showing no signs of fighting back, it is either dead or dying. 
  • Watch its movement – Sick fish are quite lethargic and sluggish.[8] Some of them will sink to the bottom of the tank and lay there. This lack of activity tells you that something has gone very wrong.
  • Sunken eyes – A fish with sunken eyes is quite close to death. Sunken eyes also show that a fish is dead. However, you should check before you conclude that the fish in question has passed away. If you observe these signs early enough, you could take steps to save the creature’s life before it dies. 
  • Cloudy eyes – Another symptom of trouble is cloudiness in the eyes. Cloudiness is normal in some fish. But if it persists, you should consult a professional.
  • Look for bloating as well as swollen eyes – These are signs of dropsy which is a severe condition that is not easy to resolve.[9] In this case, you have to separate your fish from its tank mates before it spreads this condition.[10]
  • A loss of appetite – This tells you that your fish’s condition has deteriorated. If the fish doesn’t even react when you introduce food to the tank, you should take immediate steps to remedy its ailment. 

In such situations, not only is the creature unwilling to eat, but it might be too weak to move. The presence of signs like abdominal swelling, dull colors, and open sores should concern you. But inactivity and lethargy are the worst of these symptoms.

Pro tip: Fish open and close their mouths all the time. But if the mouth remains open, your fish may be suffering from tetanus, injury, blocked throat, and more.


Fish tend to move their mouths rapidly when they lack oxygen. That could happen when toxins like ammonia, nitrites, nitrates, and carbon dioxide are starting to accumulate. Also, that could be the case when your tank is too crowded.

A dying fish may show other behaviors, such as slow movements, cloudy eyes, and loss of appetite. These warning signs mean that you should take immediate action to save your fish. Other than water replacements, you should definitely consult an expert.