Why Are My Fish Swimming At The Top Of The Tank | Causes & Fixes

If your fish are swimming at the top of the tank, it can be cause for concern. It can be confusing, especially if you’re unsure what it means. But don’t worry – there’s usually a logical explanation! 

In this post, we’ll explore some common reasons fish might swim to the top of the tank and how to remedy the situation.​ 

Inadequate oxygen is the most typical cause for fish to swim near the surface of the water column. This may occur for various reasons, including excessive water temperature, poor water quality, and so on. In addition, there are some additional factors to consider, such as illness or injury.

So read on to get the answer on the topic – “why are my fish swimming at the top of the tank”. In addition, what are the possible fixes you can apply in your aquarium to prevent that from happening!​

Why Are My Fish Swimming At The Top Of The Tank?

The introduction of oxygen into the water is accomplished by air-water contact at the surface of the fish tank. Compared to the bottom, the oxygen level on the top is relatively high. When the oxygen level drops, it is evident that fish will rise to the surface and attempt to breathe.

As a result, if you see fish gasping towards the surface, the most possible reason could be your tank is deficient in oxygen.

However, if you notice only a single fish swimming near the surface, it might not lack dissolved oxygen. Instead, this might be an issue with the fish’s gills, such as gill flukes, parasites, or injury.

On the other hand, if you discover that numerous fish are attempting to breathe at the surface or lingering at the top of the tank, something is probably wrong with their water or aquarium conditions. 

So let’s discuss the probable reasons for fish swimming at the top of the tank. 

#1: Poor Water Quality

Poor water quality results from several reasons, such as inadequate filtration, improper maintenance, overfeeding, etc. However, the consequences of poor water are significant, and it’s the most common reason that forces your fish to swim at the top. 

When the water in your aquarium has an inappropriate pH level, high in ammonia, nitrite, or nitrates, it can make breathing difficult for fish. 

When ammonia and nitrite level rises, it drives away the oxygen, resulting in gasping in the fish. In addition, fish produces extra mucus in exposure to high ammonia levels. That significantly reduces the gill’s functionality. 

More Resources: Why Do I Have High Ammonia Level In Aquarium [How To Fix]

Nitrite causes alterations in hemoglobin, which is responsible for transporting oxygen throughout the body. This increases the fish’s need for oxygen even more.

Poor Water Quality

Aquarium water high in pH may have an adverse effect on the gills of fish, which are the organs that fish utilize to get dissolved oxygen from aquarium water. If a fish’s gills are not entirely functioning, it may not receive enough oxygen to live. In other words, excessive pH levels may cause fish to perish.

The danger of ammonia toxicity in the aquarium is further increased by a high water pH. A condition known as “ammonia auto-intoxication” may develop when the water pH exceeds the fish’s blood pH.

Poor water quality can also occur because of overfeeding or dead fish. Whatever the reason, a 50% water change can alleviate the problem as a quick fix. 

However, be careful later on. Do a water quality test with a reliable test kit regularly to have clear ideas about the water parameters in your aquarium. 

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#2: Overcrowding

One of the most common mistakes people make when they bring home a new fish is forgetting that it needs more than just water to live. You see, without enough dissolved oxygen in our tanks, fish can’t survive. 

Overstocking is another reason for low oxygen levels in an aquarium. When there are too many fish in the water, that means higher pollution rates and less dissolved oxygen for breathing. 

Overcrowding forces your fish to swim at the top for getting oxygen

Therefore, you should stock your aquarium with the right amount of fish. Follow my article on how many fish per gallon for a clear idea about stocking. 

One inch per gallon is an overly generalized idea. But your stocking density may vary with fish types, fish sizes, the shape of the aquariums, and some other factors. 

More Resources:

#3: Hypoxia

Hypoxia is defined as a lack of oxygen and aeration, which causes fish to swim to the top to breathe. A healthy aquarium should have dissolved oxygen of 6-8 mg/L

There is no simple method to tell whether your fish tank has a low oxygen level other than measuring the dissolved oxygen level. However, keeping an eye on the fish’s behavior might help fishkeepers predict. 

Fish will show indications of low oxygen levels in the tank. For example, fish gasping on the surface is one of the symptoms of low dissolved oxygen levels. Others are sluggishness, difficulties breathing, lack of appetite, etc.

Hypoxia is main reason for fish swimming at the top

Low oxygen levels may also be caused by stagnant aquarium water. One of the ways oxygen enters the water is via aeration (rapid movement). Oxygen is frequently abundant at the top but does not reach the bottom because of the absence of agitation or circulation. 

The circulation of water aids in the oxygenation of the whole aquarium water volume. Therefore, if you predict your fish is swimming at the top of the tank because of low oxygen levels, increase the aeration. 

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#4: Improper Water Temperature

There is a significant connection between high water temperature and low dissolved oxygen (DO). As the temperature rises, the capacity to carry oxygen declines. Coldwater may retain far more oxygen than warm water. 

Most tropical aquarium fish thrive at temperatures ranging from 75° to 80°F. However, coldwater fish prefer temperatures below 70°F. As a result, if you are using a heater, you do not need to elevate the temperature over 80°F unless you are experiencing an Ich epidemic, in which case a maximum temperature of 86°F is advised.

Improper Water Temperature

It is crucial to keep an eye on the temperature of your water. Check more often in summer because it can change quickly and without warning! 

If you reside in a hot climate, such as the southern United States or countries in South Asia, summer may be a difficult season to get through. Using an aquarium chiller in this situation might be a logical answer.

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#5: Swim Bladder Disease

For fishkeepers, one of the most dreaded diseases is swim bladder disease. This can be a severe ailment and can often lead to death if not treated. 

Swim bladder disease can cause all sorts of symptoms in your fish, but one thing you should know is that it affects their equilibrium. This makes them float to the top or bottom and swim sideways as well!

Swim Bladder Disease

An essential component of buoyancy regulation is the swim bladder organ located in the lower abdomen. Fish exhibit abnormal swimming patterns and mobility issues when this organ becomes damaged or diseased. 

Constipation is the single major cause of Swim bladder disease. However, Overfeeding might also lead to this problem- so make sure not to feed too much food at once (or ever).

Stop feeding your fish for one or two days if they have swim bladder illness. Next, supply them with fiber-rich foods like Daphnia and blanched peas. However, treatment may not always be successful in curing swim bladder disease.

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#6: Heater Malfunction

The aquarium’s inhabitants like the warmth provided by the heater. But, on the other hand, a heater might kill your fish if it isn’t worked correctly. Likewise, if it fails and does not turn off when intended, it might cause the aquarium to overheat.

As a consequence temperature of the aquarium will continue to rise, and oxygen concentrations will plummet. This will drive your fish to rise to the surface to take in oxygen from the surrounding air. Replace the heater as soon as possible since overheating over an extended period might cause your aquarium fish to die.

Heater Malfunction

Using a separate thermometer will help you understand whether your aquarium heater is working or not. In addition, instead of a single big heater, it is suggested that two smaller heaters be used. Even if one of your smaller heaters fails, the other will not be able to cook your tank residents since it is not strong.

Another way to improve the heater’s reliability is to install a separate temperature controller, which will be independent of the central aquarium heater controller. The temperature controller is equipped with a probe and mechanism to monitor and regulate the environment’s temperature. As a result, if one controller fails, the other will catch up.

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#7: Associative Learning In Fish

The fish may swim up to the surface when it is feeding time! This behavior is not uncommon. Because some types can learn and remember what time you usually give them food. 

Learning Capabilities in Fish

They will come back down eventually on their own accord. Therefore, knowing your fish’s typical behavior may help you maintain your fish worry-free in various ways.

Fish That Can Breathe At The Top Of The Aquarium

Keep in mind that the movement of labyrinth fish should not be mistaken with that of other fish. For example, fish such as Bettas, Gouramis, and bottom feeder catfish occasionally float to the surface and gulp air via their labyrinth organ. 

When the conditions are normal, they will not remain on the surface for long. However, if you see those fish, including other fish doing the same thing, sucking air in through their open mouths, take action right away. 


After reading this article, you should know why your fish are swimming at the top of their tank. If they’re not eating or behaving strangely, there may be an issue with the water quality in the tank. 

With a little bit of research and time spent solving the problem, you can keep your aquatic friends happy for years to come! 

Leave us a comment below if you have anything more to add about these reasons or solutions.

Sujit Modak

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