Why Is My Fish Swimming At The Top Of The Tank? [Explanation]

Aquariums not only beautify spaces in our homes, but they also improve sleep quality, reduce stress levels, lower blood pressure, and bring a calming effect to kids. However, proper care and maintenance are needed to keep the fish alive.

Why are my fish swimming at the top of the tank

In most scenarios, aquarium fish swim and hang out at different depths of the aquarium. Therefore, if you find your fish swimming at the top of the tank throughout, then you should be concerned. One of the main reasons is low oxygen levels.

Here are some of the things that can cause your fish to swim at the top of the aquarium tank:

Lack Of Oxygen

The main reason why your fish may swim at the top of the tank is because of a lack of oxygen. Fish breathe dissolved oxygen. Therefore, if the oxygen level is lower at the bottom, they will swim to the top, where dissolved oxygen is always higher. Thanks to the interaction between air and water that occurs at the surface.

When you see your fish gasping for air and swimming at the top of the tank, this signifies that the level of dissolved oxygen is lower in the aquarium. Before you conclude that the issue is due to lack of oxygen, you should note if it’s one fish or all the fish in the aquarium.

If it’s one fish, it may be injured or has problems with the grills. But if all the fish in the aquarium are gasping for air, then the level of oxygen is low. You can rectify this problem by changing one-third of the water in the aquarium.


Aside from low levels of dissolved oxygen, your fish may also swim at the top of the tank because of a disease. One of the common diseases that affect how the fish swims in the tank is the swim bladder. This disease affects the fish’s equilibrium. It can make the fish swim at the bottom or top of the tank. Additionally, it can make the fish swim sideways or upside down.

Some of the major causes of the swim bladder are constipation and overfeeding. You may notice that this condition appears typically after feeding your fish, and it disappears afterward.

To avoid such a disease, you should avoid overfeeding your fish. Furthermore, you should feed it more fish, oranges, green peas, or even melon to prevent constipation.

See also: How To Treat Swim Bladder Disease In Fish

Ammonia & Nitrate

Another issue that can make your fish swim at the top of the tank is excess ammonia or nitrate or both in the aquarium. If the level of ammonia or nitrate is excess, it will make it hard for the fish to breathe.

That’s why they may be forced to gasp for air at the top of the aquarium. For this reason, you should always ensure that the PH level in the aquarium is at the recommended level.

If your aquarium has high levels of ammonia, it makes the fish produce extra mucus. As a result, it reduces the effectiveness of the gills, as it hampers them to inhale oxygen as usual.

On the contrary, excess nitrate in the aquarium leads to changes in hemoglobin, which is in charge of carrying oxygen in the blood. As a result, the fish gasps for oxygen even more.

Type Of Fish

Surprisingly, not every fish you see swimming at the top of the aquarium tank is gasping for oxygen. Some types of fish love swimming at the top of the tank. So, before you conclude that it lacks oxygen, you should learn more about the kind of fish you’ve in the aquarium.

The types of fish that love swimming at the top of the tank are hatchet fish, zebrafish, and guppies. These fish naturally swim at the surface in the wild to access food first.

However, if any of these surface fish stay in one place for long and show signs of gasping, it should indicate low levels of oxygen in the water.

Associative Learning In Fish

At times, fish swimming at the top of the tank doesn’t mean there’s a problem. Some fish types have a sharp mind and can learn and remember the time that you always feed them.

My fish is swimming at the top of the tank why?

Hence, they may link your presence in the room with feeding them. This is common if you usually feed them at a specific time.


Temperature and heating are other possible causes of your fish swimming at the top of the aquarium. Note that warm water holds less oxygen. Therefore, you should always keep an eye on your aquarium and ensure that the water temperature is fine.

It should not heat up as this will affect the level of oxygen in the aquarium. You may be forced to change water frequently in the summer.


Besides, the aquarium heater may also malfunction. If this happens, the water may overheat and cause oxygen levels to drop drastically. In such a case, you should replace the heater immediately as increased temperatures can kill your fish.


Having excess fish in the aquarium can also cause dissolved oxygen levels to drop in the tank. This is because the overstocked fish will be competing for the same air while polluting the water. Within a short period, you will see your fish swimming at the top of the tank and gasping for air.

To avoid such a problem, you should inspect the capacity of your aquarium and stick to the “one gallon per inch of adult fish” rule. This will enlighten you if you have added more fish than the aquarium can handle.

Aside from that, you will have to consider that some fish are very aggressive, while others are larger.


If you notice that the fish in your aquarium is swimming at the top of the tank, you should take quick action to save them.

Low oxygen levels, diseases, excess ammonia, dissolved gases, or even excess nitrate can suffocate your fish to death. As a result, you should find a solution as quickly as possible to avoid suffocating your beloved fish.

Besides, you should also maintain good aquarium practices such as checking the water temperature and PH levels. Also, you should regularly aerate the water by changing one-third of the water more regularly. We hope that you have found the answers that you’ve been looking for!