Why is My Cory Catfish Swimming Upside Down?

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If you’re not familiar with catfishes, you may be caught off guard by some of their behaviors.

These fish aren’t your typical guppies, so you need to get up-to-date with some of their unique behavioral displays.

Today, we will discuss Corydoras swimming upside down, which is an atypical fish behavior.

Reasons Cory Catfish Swim Upside Down

It’s always a reason for concern to notice your fish swimming upside down. Or is it?

Most fish cannot and won’t swim upside down because it goes against their very anatomical structure; fish aren’t meant to swim that way.

However, catfish distance themselves from the norm in this sense. If you notice your Corydoras swimming upside down, consider the following potential explanations:

– Normal Behavior

Yes, catfish swim upside down as part of their natural behavior. There are actual evolutionary explanations for the fish’s startling swimming pattern that describe a 35-million-old behavior.

You must first look at its anatomical structure to understand why the fish behaves that way.

Catfishes have flat bellies with side fins and mouths located underneath their heads. They are anatomically built to search for food on the substrate, using their mouths as suction cups.

This critical revelation explains why Corydoras turn upside down when near the water’s surface.

They do so for easier access to surface insects and air. Corydoras don’t typically feed at the water surface but often get there to take in atmospheric air.

They inhale air via their mouth and exhale it via the anus in a process called intestinal breathing.

And what better way to breathe at the water surface with a mouth located on the same plane as the abdomen?

Interestingly enough, there are no anatomical adaptations to upside-down swimming in catfish.

This is just a fish designed for upright swimming that tends to swim upside-down a lot, which still puzzles scientists today.

– Ammonia Poisoning

This is where things take a fast turn for the worst. Ammonia poisoning is deadly in severe cases, and if left untreated, your fish will showcase a variety of stress symptoms in this sense.

Poisoned fish will display:

  • Rapid or erratic breathing
  • Irregular swimming pattern (including upside-down swimming)
  • Dimming body colors
  • Increased mucus production
  • Red or bloody gills (a sign of advanced poisoning)
  • Secondary respiratory or skin infections, etc.

The fish will die eventually, especially if the condition is severe. Ammonia poisoning can kill fish in a matter of hours or even minutes in some cases.

Interestingly enough, Corydoras are more prone to ammonia poisoning than other fish species, and the reason for that lies in their burrowing behavior.

Cory catfish often bury themselves in the substrate, which is the crux of the problem.

Sand substrates are known for their tendency to develop anaerobic pockets, which are bubbles of gas forming due to bacterial activity.

Corydoras can pop these bubbles during their frequent under-substrate incursions, releasing the contained ammonia into the tank and poisoning the environment.

The direct contact with the trapped ammonia is often enough to kill the fish nearly instantly.

To prevent this, you need to prevent the formation of anaerobic pockets, which is a matter of regular tank maintenance.

Vacuum the substrate regularly and stir the sand every several days to prevent ammonia accumulation under the sand.

Additionally, you should remove excess food residues, dead plant matter, and fish waste to promote a cleaner and more chemically-stable environment.

Also, use a tester kit to keep track of ammonia in the water and learn when water quality starts degrading.

– Swim Bladder Disease

Swim bladder disease is a common condition among freshwater fish.

The disorder’s triggers are many, and most of them relate to food and feeding, such as:

  • Overeating – Catfish aren’t known overeaters, but they can exhibit overeating behavior, especially if there’s a lot of food-related competition in the tank. Overeating will cause the fish’s abdomen to expand, press against the swim bladder, and cause buoyancy issues.
  • Air gulping – This trigger is more common among surface eaters, so it’s uncommon among catfish. But this doesn’t mean it can’t happen. If your catfish eats at the water surface, it risks ingesting air along with the food. The air will expand the digestive cavity, causing the swim bladder to displace and cause impaired swimming.
  • Improperly-low water temperatures – The proficiency of the fish’s digestive system depends on the water temperature. If the water is too cold, the catfish’s metabolism and, subsequently, the efficiency of the digestive system will drop significantly. This will cause any additional food to stack on top of the undigested one. The result is increased stress on the digestive system and the bladder, with all the problems associated with that.
  • Physical trauma – This is rare, but it can happen, especially when keeping Corydoras with larger or more aggressive fish species. Aggressive poking can cause the fish to experience bladder trauma which will overinflate it and trigger more issues along the way.
  • Infections – Internal parasites or bacterial infections can also cause swim bladder disease in some circumstances. This happens mostly due to a combination of poor water conditions and catfish with weak immune systems due to stress or other causes. In this case, your fish requires antibiotic treatment to overcome the problem.

If you suspect your fish suffers from swim bladder disease, you need to diagnose the condition fast and accurately.

Some of the main symptoms of swim bladder disease include:

  • Curved back
  • Lack of appetite
  • Difficulties sinking or swimming to the water surface
  • Erratic swimming movements
  • Upside-down swimming for prolonged periods, etc.

If you cannot determine the fish’s condition, speak to your vet. An X-ray session should determine the condition’s nature immediately.

Do Corydoras Sleep Upside Down?

No, Corydoras don’t typically sleep upside down. These fish sleep on their sides or bellies, preferably near the substrate, in a safe area.

Others rest with their heads lower than their tails but never upside down. If your fish is in an upside-down position when resting, consider it may not be resting at all but dying. Or, at least, showcase signs of stress or physical sickness.

How to Tell if Cory Catfish is Dead?

You have several indicators to consider in this sense:

  • Lack of breathing – This is the clearest and easiest-to-check indicator. Simply look at the fish’s gills to see if they move. The breathing process is automatic, so the fish should move its gills no matter whether in a sleepy or wake state. If the gills aren’t moving, you have your answer.
  • Any movement – Fish don’t lay motionless, not even when sleeping. You should be able to detect some subtle fin movement as the fish uses them to keep its balance and position in the water.
  • Verify the eyes – Sunken eyes are the clearest indicator of a dead fish. Verify the catfish’s eyes and check the pupil’s reaction to bright lights. If no reaction is visible, the fish may have deceased.

You can also poke the fish gently with something or even remove it from the water to gauge its reaction. If the fish is inert and unresponsive, consider disposing of it hygienically.


While cory catfish can swim upside down in some situations, they should exhibit this behavior too often.

If they do, look for other signs that could explain the phenomenon. Doing so can sometimes save your catfish’s life.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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