Why are my fish swimming at the top of the tank?

In this article, we will answer the question “Why are my fish swimming at the top of the tank?” and understand what needs to be done when this occurs. 

Why are my fish swimming at the top of the tank?

Swimming at the top of the tank normally indicates a lack of dissolved oxygen in the water. This behaviour can be associated with poor water quality, lack of oxygenation, fish diseases, overpopulated tank, and temperature issues. However, it can also be a natural or learned behaviour.

How do fish breathe?

In general, fish breathe using dissolved oxygen in the water. The dissolved oxygen enters the fish body through the gills. Then, this oxygen gets into the bloodstream and the tissues.

The oxygen gets dissolved into the water in areas of interaction between atmospheric air and the water surface. Thus, water at the top of the tank is richer in dissolved oxygen than the water at the bottom of the tank. 

Air pumps and air stones increase the oxygenation at the water column, thus, the entire tank has water with dissolved oxygen. This allows fish to breathe at the water column, not only at the top of the tank.

Reasons for swimming at the surface

There are many different reasons why the fish swim at the top of the tank. The main reason for this behaviour performance is to breathe more easily due to the lack of dissolved oxygen in the water. However, issues with the water quality, diseases, and system malfunction can also provoke this practice.

Water quality issues

One of the most common reasons for fish swimming at the top of the tanks is poor water quality. Ammonia, nitrites, and nitrates levels can result in fish gasping at the surface. The high levels are toxic and reduce the gills and haemoglobin efficiency. Thus, breathing becomes more difficult to be conducted by the fish breathing system.

Issues with water hardness, salinity, and pH can also indicate potential concerns that can cause breathing problems resulting in swimming at the surface behaviour.

Lack of oxygenation

The most common reason for swimming at the top of the tank is hypoxia – or lack of oxygen and aeration. Lack of oxygenation is normally the result of issues with the air pumps or filters. If dissolved oxygen levels get under 4 ppm, action must be taken.

Lack of oxygenation is also one of the most easily solved issues with fish breathing behaviour. Water changes and an increase of water movement/current using filters or air pumps can help you to solve the lack of oxygen issue.


Some fish diseases can cause many symptoms that may result in your fish swimming at the top of the tank. Parasites at the fish gills can cause difficult gas exchanges for breathing. 

Swim Bladder is also a disease that can result in surface swimming. This disease is very common and affects fish equilibrium. The fish can also have some swimming issues associated, swimming sideways or upside down.

Controlling fish diseases and improving diet may help you to solve this issue.


An overpopulated tank can also provoke the fish to swim at the top of the tank. Overcrowded tanks cause a reduction in the dissolved oxygen level, due to a shortage of oxygen. This means there is not enough dissolved oxygen for the entire tank population. 

To solve this issue, you can opt for a bigger tank or move some fish to other tanks.

Water temperature issues

Issues with water temperature affect the concentration of dissolved oxygen (higher temperatures, less oxygen). Heater malfunction and solar exposition can increase temperature. Tropical freshwater tanks should range between 75 and 82 degrees Fahrenheit.

These issues can also be easily solved. Getting a new heater and thermostat may solve temperature issues.

Fish that naturally swim at the top of the tank

There are some other reasons fish swim at the top of the tank. There are fish that perform this behaviour naturally, as a manner to access food first, as they feed on the surface.

Another possible reason is the surface breathing behaviour of fish that have a specific organ, the labyrinth. Betta fish is a fish species that owns this organ and, thus, can obtain oxygen directly from the atmosphere.

Associative Learning in Fish

Sometimes swimming at the top of the tank, on the water surface, is not an issue and does not indicate any problem. This behaviour can be classified as associative learning.

Some fish species can associate, learn, and remember the feeding time and area of the tank. For this reason, some fish may spend specific parts of the day waiting for food at the surface. One example of fish with associative learning is the Guppies. They are even used in behavioural studies of learning processes and drug effects.

What to do when fish are at the top of the tank

In cases in which swimming at the top of the tank is an issue, some actions need to be taken. Some of them can be easily solved by improving the tank systems, heater, air pumps, and filters. Others can be solved using remedies and setting up hospital tanks, as breathing and behavioural problems caused by diseases. The water quality after water changes can be tested routinely and controlled using water kits. Additionally, it is possible to reduce ammonia, nitrites and nitrates levels by adapting and correcting food quantity, fish waste, and filtering systems.


In this article, we answered the question “Why are my fish swimming at the top of the tank?” and understood what needs to be done when this occurs.

If you have any thoughts or doubts, feel free to drop us in a comment below!

Frequently Asked Questions (FAQs): Why are my fish swimming at the top of the tank?

How do you oxygenate a fish tank?

To oxygenate a tank you can set up air pumps, air stones, and stronger filters. However, for an instantaneous solution, you can use jars to slowly pour water from some high above. The movement will increase the water oxygenation. The amount of oxygen that becomes dissolved will depend on the high and how many times the process is repeated. 

What does it mean if your fish swim to the top of the tank?

They can be having breathing issues. These issues can be associated with water quality, wrong temperature, diseases, and natural behaviour. It is also possible that the behaviour of swimming at the top of the tank is a result of the associative learning ability that some species possess.

How do I stop my fish swimming at the top? 

If you detect the behaviour is not the result of associative learning and not a natural behaviour, you can take some actions to detect and then solve any problem. Trying to oxygenate the water with better air pumps, check the heater functioning, test the water parameters and correct them. Additionally, if you detect any signs of diseases it is important to schedule a veterinarian appointment.

Is it bad if my fish is at the top of the tank?

Not necessarily, if your fish species have the natural behaviour of breathing at the surface or are known to learn behaviours it is totally fine. However, if you detect there is not learned or natural, it means your fish can be suffering from some disease, the water quality is poor, or you may have issues with tank structural systems, like heaters or pumps.

Is it possible to manually oxygenate my tank?

The easiest manner to increase the dissolved oxygen in your tank is by gently pouring water into the tank from some high. The bubbles and water movement will increase the dissolved oxygen level.

How do I know if my tank lacks dissolved oxygen?

If the dissolved oxygen is low, the fish will start to breathe at the top of the tank. This occurs because the water surface is where the oxygen level is higher. Thus, the fish breathe more easily in this area. Some other signs are breathing speed and swimming disorientation.


Why is My Fish Coming to the Surface to Breathe? https://www.petco.com/content/petco/PetcoStore/en_US/pet-services/resource-center/health-wellness/why-is-my-fish-breathing-at-the-surface.html#how-do-fish-breathe

Why are my fish swimming at the top of the tank? https://www.swelluk.com/blog/why-are-my-fish-swimming-at-the-top-of-the-tank/

Why Your Fish is Swimming At The Top pf The Tank. https://aquariawise.com/why-your-fish-is-swimming-at-the-top-of-the-tank/

What Does it Mean When Fish Swim at the Top of the Aquarium? https://aquariumsphere.com/fish-swim-top-of-aquarium/

Why Is My Fish Swimming At The Top Of The Tank? [Explanation] https://purrfectnpawesome.com/fish-swimming-at-the-top-of-the-tank/#Associative_Learning_In_Fish