What Does it Mean When Fish Swim at the Top of the Aquarium?

As an aquarist, it is hard to understand your fish’s behavior because they cannot vocalize their needs.

The only thing that can help us understand what is going on with our fish is by observing their behavior.

Routinely checking and taking care of your fish and aquarium can help you keep a check on new problems and ensure that your fish is doing alright.

You may notice that some fish varieties swim near the bottom while others remain in the middle or upper regions of the aquarium.

If your aquarium fish is spending a lot of time at the top and seems to be gasping for air frequently, it can be an indication of some problem. Some of the reasons for fish swimming at the top can be – stress, deteriorating water quality, lack of aeration, diseases, overcrowding, etc.

Keep reading to learn about the different causes in detail that can make your fish swim near the surface of the water:

What Does it Mean When Fish Swim at the Top of the Aquarium?

Fish breathe oxygen that’s dissolved in the water. The main reason for fish swimming near the surface is to breathe more easily.

The level of dissolved oxygen is higher at the top because of the interaction between the air and water that takes place at the surface.

If fish do not get sufficient oxygen in the middle or at the bottom, they tend to swim to the surface of the aquarium and you will see them gasping for air.

If it is just one fish that is swimming near the surface, it might be due to some problem with its gills or an injury.

On the contrary, if you notice more fish trying to gasp at the surface of the aquarium water, there is some problem with the water or aquarium conditions.

Let’s go through some common causes of fish swimming at the top:

Declining Water Quality

Water quality is among the most common reasons that make the fish swim at the surface.

Any issues with the levels of ammonia, nitrites or nitrates, or inappropriate hardness, salinity, pH levels can make it difficult for the fish in the aquarium to breathe.

The increased levels of both ammonia and nitrites can cause gasping in fish.

When the aquarium has a high level of ammonia, it makes fish produce extra mucus.

This reduces the functionality of the gills, making it difficult for them to inhale oxygen.

Nitrites cause changes in the hemoglobin which carries the oxygen in the blood. This makes the fish gasp for oxygen even more.

In case of any such problems, immediately do 50% water changes, top up your aquarium with filtered water, and fix any issues in the filter.

Water testing will give you a quick idea about the water quality.

Overfeeding or the presence of dead fish can also pollute the water. Determine all the causes which might be leading to the lack of oxygen in the aquarium.


If your fish appears to be grasping at the surface, it might be suffering from hypoxia.

Hypoxia is the lack of oxygen and aeration that makes fish swim to the surface to breathe.

The lack of dissolved oxygen in the water in the aquarium can cause this problem.

Test the water for oxygen levels. If the results are anything less than 4 parts per million, your aquarium needs partial water change.

Apart from that, you can also add an aeration source. Sometimes, high temperature also decreases the level of oxygen in the aquarium water.

Before topping up the water in the aquarium, let it reach room temperature.

Swim Bladder

A disease can also be a reason why your fish is swimming at the top of the aquarium. ‘Swim bladder’ is a common disease in fish.

It affects the fish’s equilibrium, making it float to the top or bottom.

It can also cause the fish to swim sideways or upside down. Constipation is the biggest cause of swim bladder in fish.

Sometimes, overfeeding can also cause this problem.

You might notice that these symptoms occur immediately after the fish eat and then go away in a few hours.

To treat this problem, you can feed your fish green peas, oranges, or melon to prevent constipation.


Adding a lot of fish can also cause low levels of oxygen in the aquarium.

Too many fish means more pollution and more need for dissolved oxygen for breathing.

An overstocked aquarium will cause a shortage of oxygen, making the fish swim to the top to gasp for air.

Check the capacity of your aquarium using the “one gallon per inch of adult fish” rule.

This will give you a quick idea if you have added more fish than the capacity of your aquarium.

That said, for a more thorough analysis, you will also have to consider other factors. Some fish are bigger while some fish varieties are more aggressive and hence need more space.

Do some research and take help from an expert to prevent overcrowding in your aquarium.

Inadequate Water Temperature

The water temperature also affects the concentration of oxygen in an aquarium.

Water with higher temperatures tends to have less dissolved oxygen than cooler water.

Keep a regular check on the water temperature with the help of a thermometer. Check the water temperature more frequently in the summer.

The ideal range of temperature for a tropical freshwater aquarium is 75-82 degrees Fahrenheit.

Prevent the temperature from rising beyond this range for the type of fish in your aquarium.

You can regulate the water temperature with the help of a chiller.

Heater Problems

Your aquarium heater can also be malfunctioning, causing the overheating of the aquarium.

If it is continually working in the “on” mode, the aquarium can heat up, causing the fish to swim to the surface.

In case of malfunctioning, the temperature can go as high as 86 degrees Fahrenheit.

Immediately replace the heater as overheating for a long time can kill your fish in the aquarium.

Associative Learning in Fish

Fish swimming to the top is not always a problem.

Some fish are capable of learning and remembering the time at which you generally give them food.

So, they might associate your presence in the room with the time to get food.

You might notice your fish swimming up to the surface or the side of the aquarium when it is time for feeding especially if you feed your fish at the same time every day.

Fish that Breathe at the Top of the Aquarium

It is good to keep an eye on the activity of your fish in the aquarium, but you should also understand that some varieties of fish breathe air from the surface naturally.

Fish like Bettas, Gourami, etc. swim to the surface of the water to inhale oxygen. For this reason, you should conduct a thorough research about the types of fish you have in your aquarium.

The last thing you want is to mistake normal behavior for an alarming problem.

If you have varieties of fish that breathe at the top, you do not need to make any changes to your aquarium.


The top of the aquarium contains the highest levels of dissolved oxygen as the air and water interact there.

When your fish experience a lack of oxygen in the aquarium, it is natural for them to swim to the surface and will gasp for air (or gulp for air).

By understanding the causes of this behavior in this article, you can make the necessary changes in your aquarium.

This will help you to provide the aquarium fish with appropriate levels of oxygen as per their needs.

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