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Why Do Fish Swim at the Top of the Tank

why do fish swim at the top of the tank

Seeing your fish swimming happily in your fish tank brings so much joy and satisfaction as an aquarium owner. However, if you find your fish constantly swimming at the top of your aquarium, it is a tell-tale sign that something is wrong inside your fish tank.

But why do fish swim at the top of the tank? It is probably because the fish is gasping for air. In normal circumstances, you can usually see the fish swimming at the bottom or in the middle of the aquarium. But when they are lingering at the top, there should be something wrong in the system which will be discussed in this article.


Reasons Why the Fish Swim on Top of the Aquarium

There are many reasons why do fish stay at the top of the tank. But one thing is for sure when they do that, they want to breathe easily. Normally, the level of dissolved oxygen is higher on the surface due to the interaction of the aquarium water and the air.

You can really never tell the specific reason why the fish stays on the top, but several factors force them to do so. These reasons are related to some issues related to some conditions in the aquarium. Let us find out below:

Water Quality

When it comes to water quality, we are referring to ammonia and nitrate level of water. These two water components, when highly concentrated in your aquarium, become toxic to the fish. Ammonia, in particular, will begin to destroy the fish mucus membrane in the fish gill.

Once the mucus membrane is destroyed, the fish gill will begin to produce new cells, making the gills swell up eventually. The swelling can cause to impede the flow of water to the gills resulting in a low amount of oxygen supply coming to the fish.

What you will do to rectify the issue is test the ammonia level of the water. This action is for you to know the level of concentration of ammonia in the fish tank water. You will need to do something to correct the issue and give the fish some breathing assistance.

On the other hand, high amounts of nitrate will also reduce the respiration capabilities of a fish. It reduces the amount of oxygen that the fish can take up. So, you need to test our nitrates as well because they can cause a lack of oxygen supply.

Once you face a high nitrate level content, my advice is to fall back on feeding the fish for a few days. Or, if you are in doubt, you can also do some partial water changes to at least reduce the level of nitrate.

Dissolved Oxygen Supply

Another reason why the fish tends to swim at the top of the tank is the lack of dissolved oxygen supply at the bottom. What you do is to make sure that your tank has the proper aeration to have the proper dissolved oxygen inside the system of the fish tank.

There are several test kits that you can use to make sure that the dissolved oxygen levels are on the right level. Always remember that everything in the aquarium uses dissolved oxygen. Whether it’s the fish, bacteria, plants, microbes, among other things, is consuming oxygen to survive.

The temperature of the aquarium can also affect the availability of dissolved oxygen in the aquarium. If the aquarium comes with a high temperature, the available dissolved oxygen will be lower. What you do is add more aeration to the system to rectify the issue.


Putting a lot of fish in your aquarium is not suitable for all the fish inside. The reason for this problem is an overcrowded aquarium can contribute to making the oxygen supply very low. Also, when there is too much fish consuming oxygen, the supply may not be enough for the demand.

The rule of the thumb on the capacity of the fish tank is one gallon per inch per fish. This rule will give a slight hint on how many fish you can put on the aquarium to prevent overcrowding and eliminate the chance of oxygen supply shortage.

Fish Diseases

When the fish is poorly sick, they also tend to stay on top of the water. Parasites that cause the sickness will invade the gills and reduce the supply of oxygen to the fish. Once they feel a low oxygen supply, they tend to stay at the top.

If you feel that the fish is sick, you can treat the whole aquarium with disease treatment. You can also help the fish breathe more by installing an airstone and an additional air pump to increase the oxygen supply.

Why do Fish Lay at the Bottom of the Tank

why do fish lay at the bottom of the tank

Apart from swimming at the top of the tank, there will be moments that the fish tend to lay at the bottom of the tank. But why do this situation will happen? The most obvious reason is that the fish you have are bottom dwellers.

Meaning it is natural that they spend the majority of their time at the bottom. These fish are also bottom feeders, which means they usually spend so much time at the bottom because that is their preference for eating.

Meanwhile, a fish may also lay at the bottom of the tank because they are probably resting or sleeping. Usually, the fish will lay at the bottom after a very active swimming session in the aquarium.

Older fish tend to also just stay at the bottom as they tend to rest more often rather than swimming actively. Newcomer fish will also stay at the bottom in the early days to hide and check on the new environment.


If you are wondering why do fish swim at the top of the tank? It is probably because of a lack of oxygen and is possibly gasping for air. This article presents several reasons why this situation happens and all the possible solutions to help the fish swim back to normal.

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