Why Is My Fish Keeping Its Mouth Open? (5 Common Reasons)

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We all know how aquarium fish often open and close their mouths repeatedly. This is quite common, especially among goldfish and koi fish.

However, sometimes, certain fish will keep their mouths open. When I first saw it myself, I got a little worried, as I didn’t know if my fish was sick or disturbed.

Fortunately, over the years, I have gained a lot of experience in this subject. Stay with me to learn why fish may behave this way and what it might indicate.

Image of a fish keeping its mouth wide open.

Why Is My Fish Keeping Its Mouth Open?

A fish may keep its mouth open for several reasons. One of the common reasons is an oxygen deficiency, which causes panting, leading the fish to keep its mouth open permanently. Other reasons include injury, obstructions in the fish’s throat, or tetanus.

It is fairly common for fish to open and close their mouths repeatedly. But what about a fish whose mouth remains open continuously?

You can blame one or more of the following:

1. Oxygen Deficiency

As you now realize, the fish’s mouth plays a significant role in breathing. It pulls water into the mouth and passes it over the gills to extract oxygen.

Usually, an oxygen deficiency causes panting. The fish will open and close its mouth repeatedly and rapidly. This behavior is quite common among goldfish.

But some creatures will keep their mouths open permanently in response to the oxygen deficiency.

Oxygen deficiencies occur because of high temperatures, overcrowding, overfeeding, poor maintenance, defective filters, and pumps, etc.

These signs usually indicate that your tank doesn’t have enough oxygen:


  • Make a 20 to 30 percent water change and pour the water from a distance.
  • If your tank seems overcrowded consider removing some of your fish.
  • Make sure the water is agitated by examining your filter and air pump (sometimes they get clogged).
  • Make sure the temperature falls within the desired range.
  • Install an air stone and place it at the center of your tank. I personally love the Hygger Aquarium Air Stone (link to Amazon), as it is extremely quiet.
Fish suffering from a lack of oxygen often occupy the upper parts of the tank.

2. Your Fish Got Injured

Fish are susceptible to jaw deformities originating from parasites, nutritional deficiencies, poor-quality water, and genetic anomalies.

Some species accrue jaw-related injuries because of their aggressive mannerisms. Firemouth cichlids are a prominent example.

They routinely struggle with dislocated jaws, especially when you pair them with neighbors such as convict cichlids with stronger jaws.

Some fish will close their mouths once their wounds heal. Others will retain a deformed jaw that remains continuously open.

Young fish that manifest deformities because of poor conditions during the earliest stages of development will also keep their mouths open even when you improve the conditions in their water.

Deformities from genetic anomalies are the worst because they compel aquarists to euthanize the entire population.

This prevents breeding pairs from producing more babies with deformities.

These signs suggest that your fish has been injured recently:

  • Your aquarium fish seem to be aggressive and frequently harass each other.
  • You may notice visible cuts and bruises on the affected fish’s body.
  • You may notice that the fins are ragged or torn.
  • The affected fish often hides and gets behind objects.


Cichlids are known to be aggressive and may attack and injure other fish in your tank.

3. Your Fish’s Throat Is Obstructed

Fish can choke. This sounds ridiculous because they live underwater. However, you have to keep two factors in mind.

First of all, fish eat through their mouths. Therefore, any object that blocks a fish’s throat will prevent the creature from eating.

Secondly, you’re not wrong in concluding that fish breathe through their gills. These organs extract oxygen from the water.

However, the creatures enhance this process by letting water in their mouths and allowing it to pass over the gills.

Therefore, obstructions in a fish’s throat are a much bigger deal than you think. The fish may keep its mouth open until you remove these blockages.

Here are a few ways to know if your fish’s throat is obstructed:

  • Your fish shows no interest in food.
  • The affected fish seems to be swollen with a distended belly.
  • Your fish swims at the top and seems to be gasping for air.
  • The fish hides most of the day.


  • Quarantine the affected fish so that it does not get disturbed by other fish.
  • Consult an aquatic veterinarian. A specialist can provide diagnosis and treatment.
  • Make sure the food you use is adapted to the types of fish in your tank.
  • Check the water parameters including pH, temperature, and ammonia. Make sure they are adjusted so your fish can heal properly.

4. Your Fish Suffers From Tetanus

People don’t associate tetanus with fish, but that is a mistake because the illness afflicts most animals. This is what you should know:

  • Tetanus originates from Clostridium tetani. You find this bacteria in the soil and intestinal tracts.[1]
  • Tetanus can infect your fish via lacerations, fecal matter, puncture wounds, etc.[2]
  • The illness can block neuromuscular transmission. Stimulating the nerves won’t produce a response in the muscles. This is why some aquarists use the term ‘Lock Jaw’ to describe tetanus.
  • The jaw of a fish with tetanus may lock in the open position.
  • J. Diamond and Jane Mellanby published a paper in the Journal of Physiology which showed that minute doses of a tetanus toxin could kill goldfish. The temperature influenced the impact of the toxin.[3]

Besides an open mouth, a fish suffering from tetanus may present additional symptoms:

  • You may notice muscle spasms and contractions.
  • The body of the fish seems to be bent.
  • The fish looks indifferent, lethargic, and inactive.
  • The colors of your fish will gradually fade.
  • Your fish will swim erratically.


If you notice the symptoms above, consult a veterinarian immediately.

An excellent YouTube video of a guy who found out that one of his koi fish had tetanus.

5. It’s Just Your Fish’s Personality

Some fish prefer to swim with their mouths continuously open.

Unless you’ve noticed concerning symptoms such as lethargy, frayed fins, and discoloration, you can ignore a fish that swims with its mouth open.

Fish have quirky personalities. Their mannerisms don’t always make sense.

Do Fish Keep Their Mouths Open When They Are Hungry?

No, fish don’t keep their mouths open when they are hungry. They will appear to gasp for air at the surface.

In other words, their mouths will open and close rapidly. They won’t stay open continuously for several seconds or minutes on end.

Common signs of hunger in fish include:

  • Aggression (the fish will harass their neighbors).
  • Rummaging through the substrate.
  • Hiding.
  • Weight loss.
  • Lethargy. 
  • Waiting at the top.
Fish do not open their mouths when they are hungry, but they may open them when they are expected to be fed.

Do Fish Keep Their Mouths Open Because They Yawn?

You can’t blame a fish’s open mouth on yawning because fish don’t yawn. Their actions may trick you into believing otherwise.

For instance, stickleback fish open their mouths and stretch their bodies in a manner that looks like yawning.[4]

However, you would be wrong if you said the fish was yawning. It wouldn’t really serve a purpose.

You could argue that fish open their mouths wide to increase the water flowing over the gills in low-oxygen environments.[5]

But you can’t classify such an act as yawning because fish don’t have lungs or a diaphragm.

How Do You Know If Your Fish Is Struggling To Breathe?

The most notable symptom is panting. The fish will open and close its mouth repeatedly.

Admittedly, this doesn’t tell you anything because panting can also occur because of stress. Gasping at the surface is a more concrete symptom.

Oxygen deficiencies can occasionally send fish to the bottom. You can blame it on stress or lethargy because the creatures don’t have the strength to swim.

But it is more common for fish to run to the top because the water in this area has a higher oxygen concentration.

After all, oxygen enters the tank through the surface. The fish will gasp at the top as a way of increasing their oxygen intake.

But again, you can blame gasping at the top on stress.

Some fish will manifest this behavior because of crowded conditions, temperature spikes, or violent neighbors below.

You can confirm your theory by investing in a portable dissolved oxygen meter. But these devices are not perfect. 

Yes, they will identify oxygen deficiencies by measuring the oxygen content. But what about labored breathing that occurs because of a disease or parasite?

Fish can gasp for air at the surface because their gills are inflamed. Perform a visual inspection. You can also consult a vet.

Send them a list of the symptoms you’ve observed if you need help identifying the illness restricting the fish’s ability to breathe.

A Picture of a parrotfish in a tank without enough oxygen.


If your fish opens its mouth and keeps it open, it could be its personality. Some fish just behave differently. That doesn’t mean there’s anything wrong with them. 

However, before jumping to this conclusion, you must first rule out more serious conditions, such as hypoxia, blocked throat, tetanus, and injuries. 

If you feel lost, don’t hesitate to consult a vet. With the help of a medical expert, you will receive a correct diagnosis and treatment in a short time.


  1. https://www.pashudhanpraharee.com/tetanus-in-animalslockjaw/
  2. https://ehs.umich.edu/wp-content/uploads/2015/12/AHreptiles.pdf
  3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1331910/
  4. https://www.technology.org/2019/11/21/do-fish-yawn-and-what-is-yawning-in-the-first-place/
  5. https://www.thenakedscientists.com/articles/questions/do-fish-yawn