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Goldfish Opening and Closing Their Mouth: Reasons & Solutions

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Goldfish are probably the most common type of fish in freshwater tanks. As a fish owner, I’ve been growing these for years. However, more than once, I noticed that my goldfish are opening and closing their mouth quite rapidly. As I was worried about their well-being, I began to research that phenomenon pretty profoundly. 

Goldfish tend to open and close their mouths rapidly due to a lack of oxygen. Passing water over their gills allows them to exploit oxygen residues presented in the water. That typically occurs in overstocked tanks that feature high water temperatures and stagnant water.

As we move forward, I will share with you a few more common reasons that may force your goldfish to open and close their mouths in a quick pace. Then, I will show you what steps you should take to prevent this issue, while probably extending your fish’s lifespan.

What Makes Goldfish Open and Close Their Mouth Rapidly?

Everyone knows that fish breathe through their gills. They extract oxygen from the water as it washes over them. However, they can also take water in through their mouths. In some cases, this habit is designed to keep the gills clean.[1]

The organs can accumulate small, unwanted particles. Given enough time, this build-up can clog the gills, inhibiting the creature’s ability to breathe. Fish can resolve this issue by essentially yawning. They open their mouths, take water in and then force it out through the gills, removing the particles. 

That is to say, there is nothing wrong with your goldfish opening and closing its mouth every so often. That being said, if this is happening continuously and rapidly, your fish is probably gasping for air. Like humans, goldfish gasp for air when they can’t breathe. 

But you shouldn’t assume that your goldfish is gasping for air only because it keeps opening and closing its mouth, especially if this behavior is recent. You should look for additional signs. For example, goldfish that struggle with an oxygen deficiency will rush towards the surface where the oxygen is more abundant. 

In some cases, they will push their mouths out of the water. If the oxygen deficiency persists, your fish will become lethargic as their health begins to fail. However, this won’t happen overnight. Fish are resilient creatures that will try to compensate for the lack of oxygen by pumping water through their gills faster (hence the gasping).

But once their situation becomes untenable, you will observe physical manifestations of their distress. Some common causes of oxygen deficiency that you can probably blame for your goldfish’s panting include:

1. Tank Size

Like most fish, goldfish cannot survive in small tanks. The small surface area will reduce the amount of oxygen dissolving into the water, creating a hostile environment for your fish. You will either suffocate them or poison them. 

This is because it is easier for waste and toxins to accumulate in small tanks. The same problem occurs in tanks that are high instead of wide. If oxygen cannot dissolve from the outside, the waste and dirt will burden your fish.

2. Overstocking

Even a large tank is useless if it is overstocked since it will suffer from low oxygen levels.[2] If you have too many fish, they will fight over the tank resources, which include oxygen. They will consume the oxygen in their environment faster than it can be replaced. 

That will lead to violence in the aquarium as your fish struggle for survival in a tank with dwindling resources. If your aquarium features many living plants, they are likely to produce carbon dioxide (especially when the lights are off).[3]

Most aquarium fish appreciate planted tanks. And plants are seemingly perfect for oxygen-deficient tanks because they consume carbon dioxide and expel oxygen. However, this only happens in the presence of a light source. 

In the dark, this process reverses, and the plants begin consuming oxygen.[4] This can turn them into a threat, especially if you have a lot of plants and the sort of bad weather that keeps the sun hidden on most days.

3. Elevated Temperature

High temperatures are dangerous because warm water cannot hold oxygen as effectively.[5] That is why aquarists that don’t own heaters hate summer seasons. The heat tends to wreak havoc on tanks whose temperature isn’t artificially controlled.

If your goldfish is seemingly gasping for air, check the heater to ensure that it is working correctly. Also, test the water with a thermometer to see if it is within the relevant range for your goldfish. Ideally, it should be between 68 to 74 degrees F.[6]

4. Stagnation

Have you ever wondered why people that are struggling with oxygen-deficient tanks are encouraged to buy filters? Well, the oxygen your fish consume comes from outside the tank. It enters through the surface of the water and works its way down.

Devices like filters and pumps agitate the surface, increasing the rate at which oxygen enters the water. In that same regard, stagnant water with no significant motion, allows less oxygen to enter.

Even though the surface typically has plenty of oxygen, without significant water movement, it won’t circulate widely enough, not in sufficient quantities. This can create an oxygen deficiency in the lower sections of the tank.

5. Waste & Toxins

Dirty tanks are always plagued with oxygen deficiencies because the accumulation of waste decreases the amount of dissolved oxygen. Some tanks are so dirty that their filters are clogged, and they have allowed algae to run amok. 

Poor water quality, on its own, can reduce the oxygen in the tank. This is on top of distressing your goldfish, weakening its immune system, and leaving it vulnerable to deadly diseases.

Also, the algae in your tank will eventually compete for oxygen with your goldfish. Some tanks are filled with waste because their owners have failed to clean them regularly. Others are dirty because the aquarists in question overfeed their goldfish. 

This trend will encourage them to produce waste in more significant amounts than usual. Such rubbish contains nitrates, nitrites, and ammonia that are poisonous to goldfish. Ammonia is especially dangerous. It will burn your fish’s gills, affecting its ability to draw oxygen from the water.

6. Stress & Diseases

Some goldfish keep opening and closing their mouths because they are stressed. Common causes of stress in fish include aggressive tankmates, starvation, crowding, and small tanks. The wrong pH and temperature could also exacerbate this situation.

Lastly, any diseases that affect the gills will compromise your fish’s ability to breathe, causing it to open and close its mouth continuously.[7] The same is true for parasites and infections. If the gills look inflamed, tattered, or deformed, you can confidently conclude that illness is to blame for your goldfish’s behavior.

How to Treat Goldfish That Are Breathing Rapidly?

If your goldfish is struggling to breathe and has communicated this issue to you by rapidly opening and closing its mouth, these are some of the steps you can take to resolve its situation:

1. Spray the Water

Get a hose and spray the surface of the water.[8] If your goldfish are near their breaking point, this approach will produce immediate results. The method agitates the water, increasing the rate at which oxygen enters the tank. 

Spraying the surface also introduces oxygen directly to the top of the tank. That is to say, as the water droplets travel through the air, they pick up oxygen, which they infuse into the water the moment they strike the surface.

2. Get a Few Airstones

An airstone is a simple but effective tool that will add oxygen to the water. The bubbles it produces can move the water, combating oxygen deficiency resulting from stagnation. For that, I recommend the Hygger Aquarium Air Stone Kit (link to Amazon). This kit features the perfect balance between quality and price. 

Keep in mind that you have to place the airstones in areas where water movement is weakest. You can also buy a filter. Besides keeping your tank clean, it will move the water, allowing the oxygen to increase throughout the tank.

3. Choose the Right Tank Proportions

Keep your goldfish in tanks of sufficient size. You should prioritize tanks that are shallow and wide. They have a much larger surface area that will permit more oxygen to dissolve in the water, preventing deficiencies from occurring in the future.

If your tank is too narrow, I highly recommend checking my recommendations for an aquarium kit. I made sure to review the precise bundle that I use at the top of the list. That bundle is highly affordable and is the perfect solution if your current tank isn’t oxygenated.

You should also consider reducing the number of fish, especially if you already know that your tank is overstocked.[9] Take some of the fish to a local fish store. Most of them will happily accept your free fish. Keep only the goldfish that your tank’s size can accommodate.

4. Feed Your Goldfish Properly

Undoubtedly, you should avoid overfeeding your fish.[10] This is the only way to reduce the amount of waste your goldfish produce. If your fish seem hungry, give them fibrous supplements that will keep their cravings at bay (such as cucumber peels).

In fact, if the amount of waste your goldfish produces worries you, try cutting back on the creature’s meals. If you previously fed it twice a day, start feeding it once a day. Give it meals in quantities it can eat in one minute or less. By reducing the size of its meals, you will bring the waste in the tank under control.

5. Maintain Hygiene

You should maintain a clean tank. If you don’t have one, get a filter and make sure to perform regular water changes. If the tank’s situation is truly dire, you might have to perform 90 percent water changes. Do this as frequently you need to restore the health of your goldfish and their tank.

Water changes will control the nitrite, nitrate, and ammonia levels. Some people believe that a filter, on its own, is powerful enough to keep their tank clean. However, that isn’t true. A filter has its place. It plays a role, and you shouldn’t attempt to maintain a tank without one. But you must also perform regular water changes to remove the components that a filter cannot tackle.

6. Eliminate Aggression

Remove violent fish from the tank. Your goldfish need to know that they are safe. You can also alleviate some of their fears by adding plants that they can use to hide. But the best and most permanent solution is to remove the aggressive fish. This will banish stress.

You may also introduce a few decorations to the tank. As long as your goldfish are out of the aggressors’ sight, things should be okay. That is also true if your goldfish are the attackers that hassle relatively smaller, docile fish.

7. Adjust the Temperature

If you already have a working heater, but the water is still too warm, consider removing the tank cover. You should also use a fan to blow across the surface of the water. If the temperature is still too high, add some ice cubes. 

However, be sure to store them in a zip-close bag before adding them to the tank. If the cubes directly touch the water, the temperature will drop too abruptly. That, on its own, could stress your goldfish while forcing them to breathe heavily.

You should also reduce the number of lights. Aquarium lighting can increase the temperature of the water. Yet, keep in mind that one way of preventing your plants from consuming the oxygen in the tank is to add sufficient lighting.

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.


Most of the time, goldfish open and close their mouths to keep their gills clean. By washing water over their body, they can remove unwanted debris and food leftovers. However, if they do so quickly, that indicates they suffer from a lack of oxygen.

To overcome that issue, you should elevate the oxygen concentration. The fastest solutions would be spraying water on the top of the tank, inserting oxygenated water directly. You should also adjust the temperature and ensure that the temperature isn’t too high for your goldfish (the ideal range is between 68 to 74 degrees F).