When observing a fish tank, you might be wondering about fish behavior. One of the habits people usually discover related to fish behavior is why fish keeps opening their mouth. Although it is a usual view in the fish tank, does it normal for the fish to do so?

Most fish close their mouth when swimming around the fish tank except for several circumstances like eating, moving fast, recovering from a fight, or yawning. If you see your fish constantly opening their mouths, there might be a problem. It is something you need to be aware of as a fish owner.

Common Reason Causing Fish Keeps Opening Their Mouths

A Koi fish open its mouth inside the fish tank.
A Koi fish open its mouth inside the fish tank.

Fish that float close to the surface while opening their mouths indicates an oxygen reduction in the aquatic environment. How much water taken into their mouths determine the air entering the gills and bodies. This condition might turn worse if the fish are struggling to breathe.

If you discover this condition, you need to test the oxygen level inside the tank soon. After you know the common reason why fish keeps opening their mouth, it is necessary for you to know the fish’s breathing behavior and how determine if their breathing struggle is due to lack of oxygen.

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Below are several fish-breathing behaviors you must know as a pet owner. 

Why Does My Fish Keep Opening and Closing Its Mouth? 

If you keep a fish in a tank and find it opening and closing its mouth often, it shows a problem in its living environment. It is because this breathing habit is uncommon for the majority of fish. It usually indicates the low levels of oxygen in the water.

Fish breathe using their gills. When fish opens and closes its mouth, it enables more water containing oxygen to go through the scales and continue to enter the body. If the water in the tank containing low oxygen levels, the fish mouth keeps opening and closing more frequently.

How Frequent Fish Open and Close Their Mouths? 

Fish usually move their mouth to open and close approximately 50 times within 1 minute. Making the water in the tank colder, it can hold more dissolved oxygen. Hence, if you wonder why fish keeps opening their mouth, you may also check the water temperature inside the tank.

Fish who live in an aquatic environment with a temperature under 20 degrees Celsius do not open and close their mouth too often. You might find this breathing habit is more frequent for tropical fish in the warmer water temperatures; between 24 to 27 degrees Celsius.

Fish Struggle to Breathe

It is common for fish to breathe rapidly when they move into a new community tank. Fish just transferred into a new environment may shock and increase their stress level. This condition causes difficulty in breathing which makes them look painting.

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Another condition that can explain why fish keeps opening their mouth is harassment from other tank inhabitants. Hence, you must check the condition of the fish in their new tank if you discover a case of fish opening mouth frequently.

Fish Breathe on Top of the Aquarium

Fish frequently swim close to the surface to enable them to breathe easily. It is because the water’s surface contains more dissolved oxygen. It rarely happens unless fish face any difficulty in breathing. The water quality can be another culprit why fish keeps opening their mouth.

Poor water quality can contribute to this breathing habit. You must test the pH level of water inside the tank to maintain it remains safe and comfortable for fish to live. Besides, water may also contain excessive ammonia that can intoxicate fish and cause them dead.

Always Make Sure the Fish Tank Have Enough Oxygen

Goldfish with its mouth wide open.
Goldfish with its mouth wide open.

You can check the water regularly with an oxygen meter to know that the oxygen levels remain good and support fish life. Using an oxygen meter will give you exact information about the oxygen level. However, fish behaviors can help determine the water quality and conditions.

If the fish keep swimming near the surface with their mouth open to get more air, it possibly indicates a lack of oxygen in the fish tank. Another behavior that shows this lack of oxygen in the water is the fish swimming close to the filter output.

The primary reason why fish keeps opening their mouth is due to the low amount of oxygen in their living environment. This breathing habit is the effort of the fish to take more water into their gills so their bodies can get enough air for circulation.

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If you discover this breathing habit happens more often, then it is time for you to check the oxygen and ammonia levels in the fish tank.

How Do Fish Gills Take Oxygen from the Water?

The function of the gills is similar to the lung. They are branching organs that you can see on both sides of the fish head. Fish gills comprise countless petite blood vessels that absorb oxygen from water.

It happens simultaneously with emitting carbon dioxide and ammonia as the side product of their respiration. When fish open and close their mouths, water moves toward their gills. It allows blood in their tiny blood vessels to hold the dissolved oxygen in the water.

Then the blood goes through the fish’s body. This blood circulation process is crucial thing to keep the fish alive. If not, they will indicate it through their breathing habit. It is why fish keeps opening their mouth.