Why Is My Betta Fish Opening His Mouth? (9 Reasons with solutions)

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Betta fish have long been one of the most popular fish in the aquarium hobby.  Each year they are sold in their millions around the world. Unlike so many other fish, Bettas become true pets, even part of the family.  Because Bettas are loved so much, we often worry about them when we think something is wrong.

In this article I look at 9 reasons a Betta may be continually opening its mouth, and what the solution to each issue may be.

The most common reasons a Betta continually opens its mouth wide are;

  • To clean its gills
  • Display of aggression or dominance
  • Betta can’t breath
  • Aquarium water is too hot or too cold
  • Ammonia levels are too high
  • Chlorine or Chloramine in the water
  • He is hungry
  • There is a contaminate in the water
  • It is just his personality

To clean its gills

Although they live underwater, Betta fish still need to breathe oxygen to survive.  Unlike us, they don’t breathe air, they have to extract oxygen from the water they live in.

Betta fish use their gills (the slits on the side of their heads, just behind their eyes) to extract oxygen from the water.  They pass water across various plates in their gills and as they do they absorb oxygen directly from the water and expel carbon dioxide.

Occasionally, a Bettas gills may become dirty due to food getting caught in them or because of too many ‘solids’ floating in the water.  

It is normal for a Betta to want to flush water through his gills now and then, but if he is doing it continually it may indicate there is a problem.


If your Betta is continually trying to flush water through his gills, take a moment to observe him.  Can you see a blockage physically in his gills, maybe a small piece of a plant leaf or some food? 

If you can see a physical blockage, carefully catch the Betta in a small net and see if you can remove the blockage with a pair of tweezers, taking care not to actually touch or damage the gills.

If there is no blockage, is the water dirty or does it have detritus floating in it?  If it does, check the filter is working, and if necessary, clean the filter so it runs more efficiently.

If the tank water appears dirty, consider changing about 50% of the water to reduce the amount of dirt and detritus in the water.

A display of aggression or domiance​

Betta used to be known as Siamese Fighting fish.  In the Far East Bettas were pitched against one another, fighting to the death.

Bettas are naturally territorial and aggressive fish, which is why we only ever keep one male Betta in a tank.

Modern Bettas are now so far removed from their wild cousins, that the majority of them are far more placid than previous generations. However, they do still have a territorial streak.

Sometimes a Betta will be able to see another Betta from his tank, or maybe his own reflection in the glass, and that will be enough to make him become aggressive and territorial.  

If your Betta has his mouth wide open, is flaring his gills, and waving his fins around, it is probably a show of dominance or aggression.


If your Betta can see another Betta in a different tank, or even another fish that he perceives is another male Betta, consider placing a physical barrier between him and the other fish.  A piece of card between the two tanks will often do the job.

If the Betta can see his own reflection, it can be a little more tricky to resolve.  Try adding some more decorations to the tank.  Planting live or fake plants around the edges so the Betta can see less of the sides of the tank.

The video below shows a Betta gill flaring and opening its mouth in a show of dominance.

Betta can’t breathe​

As mentioned above, fish breathe by absorbing oxygen directly from their aquarium water.

When conditions are bad, the oxygen levels in the tank can drop to the point fish struggle to absorb enough oxygen from the water.  

Betta fish belong to a family of fish called anabantoids. Anabantoids possess a lung-like labyrinth organ allowing them to breathe air.

When oxygen levels in the aquarium get very low, a Betta may ‘gulp’ at the surface in an attempt to draw air into its labyrinth organs.  This gulping action may come across as the betta continually opens its mouth.


If a Betta is continually opening its mouth due to a lack of available oxygen in the aquarium, consider adding an airstone connected to an air pump (this one from Amazon has always worked well for me).

When we add an airstone to an aquarium, the constant stream of bubbles rising to the surface causes additional surface movement, which improves the exchange of gases between the water and the air.

Essentially an air stone = more oxygen in the water

Aquarium water too hot or too cold​

Bettas are tropical fish. They have evolved to live in warm water.  Often Bettas are kept in small fish tanks which have no aquarium heater in them.

A small volume of water without an aquarium heater can suffer from dramatic temperature swings.

In most homes across the US, the nighttime temperature is lower than the daytime temperatures because most of us do not bother heating our homes during the night.

The result of having no heater in the aquarium is the temperature of the water in the Betta tank will plummet at night.

One way a Betta may be showing that he is struggling because the water is either too hot or too cold is by continually opening and closing his mouth.


Solving this problem is really easy, just add an aquarium heater.

Bettas like their water to be between 74°F and 78°F (23°C – 25.5°C) so by setting the heather to about 76°F (24.5°C), your Betta will be happy day or night.

Ammonia levels are too high​

When a Betta fish goes to the bathroom, its waste is really high in ammonia.  Unfortunately, ammonia is very toxic to fish.  Even at a relatively low level, ammonia can poison and kill a Betta very quickly.

Luckily, we can use aquarium filters to reduce ammonia.  

Aquarium filters work by drawing in water, passing it over a form of media (usually sponge), and returning the water back to the tank.

Whilst the sponges remove physical waste, they also remove chemical waste like ammonia.  

The sponges inside an aquarium filter are covered in bacteria.  This good, or ‘beneficial’ bacteria as it is sometimes known, has the job of converting the highly toxic ammonia firstly to the less toxic nitrite, then again to the even less toxic nitrate.

When Bettas are kept in aquariums without a filter, there are very few beneficial bacteria present to convert the ammonia to the much safe nitrate.  Subsequently, the ammonia will build-up to the point it actually burns the Bettas gills, often turning them black.  At this point, the Betta will struggle to breathe and maybe opening and closing its mouth in an attempt to pass more water through its gills.


If your Betta is continually opening and closing its mouth due to ammonia poisoning, you firstly want to change a large portion of the water.  50% to 75% should be changed immediately (taking care to dechlorinate the water if necessary – see below).

A large water change will instantly reduce the amount of ammonia present in the water.

The longer-term solution is to fit a good quality filter to your Bettas aquarium.  A filter is a must for a Betta tank, no matter what else you might have read on the internet.

A Betta that is made to live in water with high amounts of ammonia present will not only suffer from ammonia poisoning but can also be more susceptible to Ich (Whitespot) or may die because it is so stressed.

Chlorine or Chloramine in the water​

Carrying out regular water changes on our Bettas tanks is an essential part of maintaining a clean, healthy environment for our fish.

If like myself you live in an area where the water company adds chlorine or chloramine to the mains water supply to keep it safe to drink, you might find that after carrying out a water change, your Betta spends a lot of time opening and closing its mouth.

Both chlorine and chloramine are toxic to fish.  The chemicals burn their gills and affect the scales and slime coat.

Your Betta is opening and closing its mouth in an attempt to move more water over its gills.  Unfortunately,  the more water the Betta passes over its gills, the worse the burning from the chlorine or chloramine will become.


When carrying out a water change in an area with chlorine or chloramine in the water supply, it is essential to use a good quality dechlorinator.  Dechlorinator breaks the chemical bond in the chlorine or chloramine, making the chemicals less toxic to the fish.

For many years now I have favored Prime which is made by Seachem.  

Betta is hungry​

This one might seem obvious, but sometimes a Betta will constantly open and close its mouth because it is hungry.

Betta fish are true carnivores that in the wild live on a diet of antsaphids, flies, and waterborne crustaceans. Although they will take commercial pellet foods in our aquariums, sometimes those pellets are not satisfying enough for the Beta.


Trying adding meaty foods to your Bettas diet.  Bettas will love to eat frozen or live bloodwormsdaphnia, cyclops, or Mosquito larvae.  

Personally, I use A LOT of frozen foods.  I try to feed my Bettas something live or frozen at least once every day, with bloodworms being my most often fed frozen food.

There is a contaminate in the water​

If your Betta is continually opening and closing its mouth, but you haven’t carried out a recent water change and none of the other potential issues on the list seem likely, maybe a contaminate has entered the water.

In our homes, there are countless products that can find their way into our aquariums, especially if we keep our Bettas in open-top aquariums. 

Room sprays, deodorants, bug sprays, and even oils and products from our hands when we put our hands into the aquarium. 

It can be almost impossible to know what or if a contaminate has entered the aquarium water as there will probably be no physical sign of it, although a slightly oily film on the surface may give some clue.


If you believe an external contaminate may have entered your Bettas aquarium, start by carrying out a large water change.  I would probably consider changing anywhere between 50% and 75%, just to be safe. 

After the water change, double-check your Bettas filter is working properly so any physical contaminants will be filtered out.

To prevent contaminants from entering the water in the first place, add a tight-fitting lid to your Bettas aquarium if you don’t already have one, and always wash your hand, without using detergent or lotion, before putting your hands into your Bettas tank.

It’s just his personality​

If none of the options on the list fit with your Betta or Betta aquarium, it is perfectly possible that constantly opening and closing its mouth is just his personality.

Just like some dogs bark a lot and some cats continually purr, some Bettas just like to open and close their mouth all the time.  There’s nothing you can do about it, just enjoy your Betta being himself!

About the Author

I’ve been keeping, breeding, and showing tropical fish for nearly 30 years. Over that time I’ve done it all! I’ve had great success and I’ve made some really foolish mistakes (like the time I bought an Asain Walking Catfish). Read more…
Richard James

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