Corydoras are bottom dwellers, so when they swim to the surface or show unusual swimming behavior, you’ll probably think something isn’t right.

And you’re right!

Corys find their food at the bottom of the tank and will rarely swim past the middle of the tank. So, what does it mean when a cory catfish swims at the surface of the water in your tank?

It is normal for cory catfish to swim at the top of the tank once in a while to get oxygen and regulate swim bladder pressure. However, spending extended periods at the surface could indicate low oxygen in the water, ammonia toxicity, or swim bladder problems in your cory.

Cory Catfish at The Top of The Tank
Cory Catfish at The Top of The Tank

Reasons corys swim at the top

If you are sure that the water is well supplied with adequate water flow, there is no need to worry since a normal cory will pop at the top once in a while. They can take up to 45 breaths per hour through aerial positioning. They also like gulping air from the water’s surface after meals.

However, below are other issues that can make corydoras come to the surface of the water.

Low oxygen in the water

If the gulps are too frequent or the fish swim too much on the surface, it is probably stressed due to low oxygen levels.

Oxygenate the water and see if the cory will spend more time at the bottom of the tank. 

The best way to oxygenate your aquarium is to install a bubbler or a pump. Despite the cory catfish being a hardy survivor that can get to the surface to catch breaths, it is critical to oxygenate the water to avoid other complications like illnesses or stress.

Swim bladder problems

A swim bladder disorder may also cause a cory catfish to stay at the top of the tank longer than usual. It can also manifest as vertical death hang in fish.

A deflated swim bladder reduces the fish’s buoyancy while a swollen swim bladder increases its buoyancy, forcing the cory to stay at the top of the tank. You may notice the fish struggle to swim back down to the bottom of the tank if the pressure regulation in the bladder is a problem.


The best fix is to take your cory to a vet for treatment. The veterinarian will treat the fish either surgically or by immersing your fish to correct the pressure problems that are forcing her to float involuntarily.


Swimming at the top for cory catfish is rarely a sign of stress. However, swimming at the top for too long may significantly cause the fish stress.

Stressed corys may opt to float as fanning their fins for too long to stay afloat may also cause fatigue.


Check the water quality and oxygen levels to ascertain why your fish prefers swimming at the top. Notably, natural oxygenation of water occurs more easily on the surface where the water and air meet. Thus, the fish may prefer swimming on or near the surface.

Swim bladder pressure regulation

Staying at the bottom of the tank means that the swim bladder pressure reduces, making the cory heavier than the water around it. Swimming and floating become difficult until the pressure is increased.

The fish will go to the top of the tank to regulate the pressure and increase its buoyancy. This is normal behavior in corydoras as it helps the fish balance the pressure for proper swimming.

Increased ammonia toxicity

Ammonia toxicity can kill fish overnight if not fixed on time. Your cory swimming at the top of the tank could be a sign of distress caused by ammonia toxicity.

Since ammonia reduces oxygen concentration and alters water pH, your cory might swim to the surface frequently to get oxygen.


Test the water for ammonia and pH. Next, perform 25-50% water change to fix the toxicity in the water. You can also use an ammonia detoxifier to purify the water quickly.

Does a cory catfish float or sink when dead?

Fish will automatically sink when dead. However, it is common to find dead fish floating in the water. Floating is a normal occurrence that usually follows the swim bladder filling up with gas.

When decomposition begins, gasses are produced. The gasses usually fill all body cavities, making the fish float naturally because gasses are less dense than water.

Therefore, if your cory dies in the tank, it will float at the top after some time.

Ensure that you remove the dead fish from the aquarium using a fishnet. Also, ensure that you purify the water using a canister filter to eliminate impurities that the dead fish may leave due to decomposition.

What if the cory doesn’t come to the top?

Corydoras often come to the top to catch some breath. The behavior is most common in oxygen-deficient water. However, if your fish rarely comes to the top, your water quality is likely good.

Consistent movement and action from your cory catfish indicate they are healthy and alive. However, if they stay motionless underwater for too long, you should be worried as it may signal a problem. Try to disturb the fish to see if there’s a reaction.

However, you do not have to worry if they do not come to the top but maintain significant activity underwater. The fish are probably getting sufficient food and oxygen underwater and might not want to get to the top.

A new tank may make fish uncomfortable, especially if the oxygen levels are not adjusted properly.


If you establish motion and your cory catfish maintains swimming at the top for too long, do not panic. Take the right steps to maintain the water quality. You may have to switch on the canister filter to clear the water of impurities. Also, turn on the bubbler or pump to oxygenate the water.

If the fish has suffered a swim bladder complication, allow it time to rest. It will most likely rest at the bottom as it recovers. You can turn off the aquarium light to give the fish a sense of security.

Recovery from a swim bladder complication can take a few hours. Your cory catfish will be back to normal if the complication is not serious.


  • University of Chicago Press Journals: The Transit Cost of Aerial Respiration in the Catfish Corydoras aeneus (Callichthyidae)

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