Fish Opening and Closing Mouths – What Could Be Causing It?

Alison Page

Alison Page


fish opening and closing mouth

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One of the main reasons for getting into the fish hobby is the relaxing feeling you get from simply watching your fish gliding through their beautifully aquascaped environment.

Although all fish species behave slightly differently, one common behavior that you’ll observe in all fish is that they open and close their mouths while swimming around in their tank.

But why do your fish open and close their mouths? How often should they open and close their mouths? And is that a sign of disease in your fish?

Generally, fish open and close their mouths rapidly because of a lack of dissolved oxygen in the water.

Read this guide to find out all you need to know about why your fish open and close their mouths!

Why Do Fish Open and Close Their Mouths?

There are several reasons why your fish open and close their mouths.

However, the overriding reason for fish to behave that way is that they are trying to extract more dissolved oxygen from the water in their tank. That’s usually a sign of oxygen depletion in the water.

It’s a Fish Thing!

Although a fish opening and closing its mouth can be a sign of something more serious, it’s usually just normal, natural behavior.

So, if your water parameters are correct for the fish species you keep, and the tank is clean and well-maintained, you have no cause for concern.

For example, fish breathe through their gills. Sometimes, the gills can become slightly obstructed, especially if the fish are rummaging around in the substrate foraging for scraps of food.

To clean the gills, the fish pull water through the gills and expel it, often several times in succession.

Fish, such as bettas and gouramis, are labyrinth breathers. That means these fish need to breathe atmospheric air at regular intervals to supplement the dissolved oxygen they derive from the water.

Your Fish Are Hungry

Many pet fish get used to their owners approaching the tank to feed them. My goldfish are pretty intelligent creatures, and they know what time they’re usually fed.

At feeding time, the fish hang around the top of the water, waiting for their meal. When I come up to the tank, they begin opening their mouths in readiness for the food to be dropped into the tank.

So, it could be that your fish are simply waiting to be fed!

Providing that there are no signs of distress and your water is clean and within the correct parameters, your fish might be hungry.

That said, don’t be suckered into feeding your fish whenever they ask you! Overfeeding ultimately leads to poor water quality and obese fish, neither of which are good.

Stick to a regular feeding regimen, and give your fish only what they will eat within a couple of minutes, several times a day if possible.

If you’re not around to feed your fish, consider investing in an automatic fish feeder to do the job for you.

Oxygen Deprivation

If your notice your fish opening and closing their mouths rapidly, that can indicate an issue with oxygen availability in the water.

Basically, the fish breathe more rapidly to pull more water (and oxygen) across their gills. You’ll also notice your fish sticking to the top of the tank where there’s more dissolved oxygen available.

  • You need an efficient filter system in your tank to agitate the water surface and increase oxygen circulation throughout the environment.
  • Avoid overcrowding your fish tank. Overstocked tanks lead to poor water quality, stress, and oxygen deprivation for the fish.
  • Keep the water clean by carrying out regular water changes, maintain your filter media properly, and monitor the water quality for increases in ammonia and nitrites.
  • Adding a bubbler or an air stone powered by an aquarium air pump can help to agitate the water’s surface, increasing dissolved oxygen in the water.

You can see that poor water quality is immediately connected to a lack of oxygen content in the tank.

Poor Water Quality

fish opening and closing mouth

Poor water quality can effectively suffocate your pets. That’s because dirty water contains less oxygen than your fish need.

If the water is oxygen-deprived, the fish will gasp for air, usually around the surface or close to the filter output where there’s more available dissolved oxygen.

Fish produce waste throughout the day and night. If fish waste is allowed to accumulate, it will gradually decompose, producing toxic ammonia and nitrites that will ultimately poison your fish.

To keep the water clean and healthy for your pets, you need to perform weekly partial water changes and remove organic waste matter from the substrate with an aquarium vacuum cleaner.

Be sure to test your fish tank water every week with an aquarium water testing kit. That way, you can spot any early warning signs before your water quality deteriorates to such a point that your fish’s health is affected.

Stressed Out

Just like people, fish breathe more rapidly when they’re stressed out.

So, if there’s no problem with the water quality, your fish aren’t hungry, and there’s plenty of dissolved oxygen in the water, stress is your next most likely culprit.

Take a look at your fish’s environment for likely causes of stress:

  • Have you introduced an aggressive tank mate recently?
  • Does your tank have plenty of planting, rocks, driftwood, and other hiding places where shy fish can take refuge?
  • Have your fish outgrown their tank? Most fish you buy from pet stores are juveniles and can double or even treble in size within just a few months.

If you have new fish that are panting, it could be that traveling from the pet store to your home has temporarily stressed the fish. In that case, you would expect the fish’s behavior to settle down and return to normal within a few days.


If your fish suddenly start opening and closing their mouths for no obvious reason, a disease could be the cause.

Some diseases affect the fish’s gills, compromising their ability to breathe normally.

In the case of gill-related diseases, you should be able to see redness around the gills, and the sick fish might also have a poor appetite or display other unusual behavior or signs of disease.

Temperature Shock

Did you know that both cold-water and tropical fish can be subject to temperature shock if the water in their tank is too hot or too cold?

In fact, there’s more dissolved oxygen in cooler water, so if you have cold-water fish, such as goldfish, they won’t be breathing rapidly because their tank is too cold.

However, if the water in the tank is too warm, the fish will gasp for air since the levels of dissolved oxygen in the water are too low for those oxygen-hungry fish.

To guard against erratic temperatures in your fish tank, be sure to check the water temperature with a proper aquarium thermometer regularly.

We recommend using a digital thermometer that you can fix to the side of your tank. That way, all you need to do is glance at the thermometer every day to check that the water temperature levels are correct for your fish.

If the water is too warm or too cool, that could indicate a problem with your heater thermostat or a heater malfunction. In that case, you need to buy a replacement heater asap to prevent your fish from going into temperature shock.


In this part of our guide, we answer some of the most commonly asked questions about why fish open and close their mouths.

Q: Why Are My Fish Rapid Breathing or Panting

A: Panting or rapid breathing is pretty common when introducing new fish to a community tank.

Often, the shock of transportation and introduction to a strange environment stresses the fish, causing them to breathe more rapidly or pant.

Bullying in a community setup can also cause panting, so observe the behavior in your tank if you notice your fish panting frequently.

Q: How Many Times Do Fish Open and Close Their Mouths in One Minute?

A: A fish opens and closes its mouth up to 50 times in just one minute!

Cooler water contains more dissolved oxygen than warm water, so cold water species, such as goldfish, don’t typically open and close their mouths as frequently or as much as tropical species.

Q: What Does It Mean When Fish Keep Opening and Closing Their Mouths?

A: When a fish opens and closes its mouth repeatedly and rapidly, that can indicate that oxygen levels in the water are too low.

Fish breathe through their gills, so opening and closing their mouths enables more water to flow over the gills, taking more oxygen to the fish’s body.

If the water doesn’t contain sufficient oxygen, the fish will need to breathe more rapidly to compensate.

Q: Why Are My Fish Breathing at the Water’s Surface?

A: Dissolved oxygen levels are higher the closer you get to the water’s surface, where the water combines with atmospheric air.

Unless your fish are struggling to breathe, that behavior is not a cause for concern. In fact, some fish species, such as bettas and gouramis, habitually visit the water surface to breathe through their labyrinth organ.

Poor water quality can also cause your fish to breathe more rapidly, so be sure to test the ammonia, nitrite, and nitrate levels in the tank regularly.

Q: How Do I Know if My Aquarium Contains Enough Dissolved Oxygen?

A: If your fish hang around at the water’s surface, apparently struggling to breathe, that’s a sign that the water doesn’t contain enough dissolved oxygen.

Fish congregating around the filter outlet is another sign that oxygen levels in the water are too low.

Ideally, your fish should be swimming around in their preferred area of the water column without panting or breathing rapidly. That shows that the fish are getting the oxygen they need from their environment.

Q: How Do I Give My Fish More Oxygen?

A: The best way of providing your fish with more oxygen is to run a high-quality, efficient filtration system and an air pump.

Filter systems aerate the water by increasing water movement. That helps to circulate dissolved oxygen around the tank, including along the bottom and close to the surface where oxygen exchange happens.

An air pump produces oxygen-filled bubbles that can help to increase oxygen in the water. However, most of the bubbles burst when they reach the surface, so an efficient filter is more effective.

Q: Can You Have Too Much Dissolved Oxygen in a Fish Tank?

A: Although your fish need plenty of dissolved oxygen to remain healthy, it is possible for them to have too much of a good thing.

If you over-oxygenate your fish tank, you risk causing gas bubble disease, where the fish exhale large quantities of carbon dioxide.

When that happens, the fish’s skin bubbles, and its eyes protrude. The disease can also be caused when concentrations of nitrogen in the water increase rapidly.

If you notice that happening, you must test the aquarium water immediately for excess levels of ammonia.

Final Thoughts

Did you enjoy our guide to why your fish open and close their mouths? If you did, please share the article before you go!

Some species of fish habitually breathe at the water surface through their labyrinth organ, and others flush water through their gills every so often to clear an obstruction, rather like you might blow your nose sometimes.

However, if your fish suddenly start opening and closing their mouths rapidly, that’s usually a sign of a problem in the fish’s environment or could be symptomatic of a disease.

Why did your fish start breathing rapidly? How did you solve the problem? Tell us in the comments box below.

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