Why Are My Corydoras Keep Dying?

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It’s never easy to find your Corydoras dead. This is a fish you’ve invested time, money, work, and emotions into, and seeing it go can strike a cord more often than not.

But what if all your Corydoras keep dying, including any new ones you might get along the way?

It’s time to dig deeper and uncover the reasons why your Corydoras keep dying without no apparent cause.

Top Reasons Your Cory Catfish Keep Dying

If your Corydoras appear sick, stressed, and start dying, consider the following potential causes:

1. Poor Water Conditions

Corydoras rank as bottom-dwelling scavengers, which is why most people think they can endure and adapt to any water conditions.

This is obviously false. Corydoras require fresh and clean waters to thrive, so you need to adopt a rigorous cleaning and maintenance routine to keep your catfish healthy.

If water conditions are suboptimal, Corydoras will display signs of stress such as low appetite, hiding behavior, lethargy, etc. It’s not uncommon for catfish to die in such conditions, often even without warning.

You should always stick to a steady maintenance routine to keep the water in optimal conditions.

Weekly water changes are also necessary in most cases.

2. Ammonia Poisoning

Ammonia poisoning is probably the most widespread cause of death among Corydoras.

There are 3 primary instances of ammonia being responsible for most Corydoras deaths:

  • New Tank Syndrome (NTS) – This issue occurs in uncycled tanks that haven’t been chemically and biologically stabilized. Many aquarists choose to cycle their tanks with their Corydoras inside, which can prove fatal. The fish are meant to generate the raw materials for ammonia production, which kickstarts the cycling process. The resulting ammonia will serve as food for new-forming colonies of denitrifying bacteria, which will eliminate the chemical completely. The problem is that Corydoras are very sensitive to ammonia, so they might not survive the cycling process. You can easily circumvent this problem by always going for fishless tank cycling and only adding your catfish once the environment is stable and safe.
  • Lack of maintenance – We’ve already discussed the dangers of poor water conditions but haven’t discussed the impact of ammonia and what triggers it. Ammonia is the direct result of decaying organic matter, which can be food, other dead fish, fish waste, plant matter, etc. The problem is even bigger in a community setup where you have many fish-producing waste and food residues that your catfish won’t be able to eat completely. Consider a more thorough cleaning routine and perform regular water changes, depending on the tank’s size and how many fish you have.
  • Anaerobic pockets – Anaerobic pockets are the invisible threat that are mostly associated with sand substrates. Water cannot circulate freely through the substrate due to the sand’s compact nature, which leads to anaerobic pockets, sometimes deep into the substrate. These are nothing more than empty spaces filled with bacteria, ammonia, and nitrites which represent an environmental hazard. Since Corydoras are substrate diggers, it won’t take much for them to pop an anaerobic pocket and release the ammonia into the water. This can result in severe ammonia shock and swift death in most cases.

Managing anaerobic pockets comes down to adopting a thorough substrate maintenance routine.

Stir up the substrate occasionally and vacuum it regularly as part of your maintenance routine. This should be enough to prevent the formation of anaerobic pockets moving forward.

Also, have a tester kit nearby, just in case. It doesn’t hurt to test water parameters occasionally, especially if you notice your fish exhibiting signs of stress or physical discomfort.

3. Chlorine Poisoning

Chlorine poisoning is another common problem in most fish tanks. The issue is more prevalent in Corydoras tanks because these fish are more sensitive to the chemicals than many other species.

But how exactly does chlorine reach your fish tank?

Consider the following possibilities:

  • During water changesTap water is to blame here. Many inexperienced aquarists use tap water during water changes. It’s readily available, easy to use, and doesn’t seem to carry any health risks. Until you learn that tap water is often filled with chlorine which, while harmless to us, humans, is poison to fish. The fix is simple: avoid tap water when performing water changes. Or you can dechlorinate the water prior to using it. Boiling it for about 15-20 minutes, allowing it to breathe for several hours, and using a dechlorinator for safety and added minerals are key strategies in this sense.
  • When cleaning decorations and equipment – Occasional generalized cleaning is necessary to prevent the accumulation of algae, detritus, and other organic residues. Using tap water in the process will bring about all of the dangers we’ve already discussed. You can either use tank water during cleaning, dechlorinate the tap water before use, or simply use chlorinated tap water, but let the decorations dry out completely before mounting them back into their place. The chlorine will dissipate naturally with time. Pro tip: don’t use chlorinated water to clean your filter media, as the chemical can destroy the colonies of denitrifying bacteria colonizing the component.
  • When cleaning plants – You need to clean and disinfect new live plants that you plan to add to the tank. This is to remove any dirt, bacteria, viruses, and even worms and snail eggs that may hitchhike their way into the tank. Using chlorinated water is bad because the chemical can transport itself into the tank water. Dechlorinated water is the answer to all your problems.

Furthermore, I recommend chemical filtration to decrease the risk of chlorine or ammonia poisoning and keep the ecosystem cleaner and more balanced.

4. Sickness or Parasite

Corydoras can experience various health problems for various reasons. These include improper tank conditions, high stress, skin injuries than can get infected, etc.

Hardy, healthy, and mature Corydoras are more resilient in this sense, but juveniles and stressed fish are more prone to health issues.

You can tell that something’s wrong with your cory catfish by assessing the fish’s behavior.

If your Corydoras shows signs of stress or has visible skin injuries, discoloration, or white patches near the gills or mouths, consider quarantine and treatment.

Quarantine is essential in this sense to prevent the condition from spreading and provide you with more control over the treatment process.

5. Aggressive Tank Mates

Corydoras are peaceful and timid fish that don’t like to get into scuffles with other tank occupants. Aggressive and overly curious fish can bother them frequently, forcing the catfish to retreat into their safe spaces.

If the situation becomes routine, your catfish will become stressed and experience a lower immune system, leaving them open to infections and various health problems.

Not to mention, overly aggressive and territorial fish can even kill or eat your catfish. If that doesn’t happen, the constant attacks can cause physical damage, leading to secondary infections and death.

To prevent this problem, always pair your catfish with equally peaceful fish species, preferably mid-to-top dwellers, to prevent territorial behaviors.

You can also increase the tank’s size, add more plants, and provide your catfish with a variety of hiding areas near the substrate. This allows them to hide if things get hot.

6. Overcrowding

Sometimes, even the calmest and most easy-going fish species can get irate and cause a ruckus when overcrowded. Overcrowding is a frequent problem in fish tanks because people always want more fish in their aquariums.

The problem is that overpopulation can easily become deadly, even if there are no territorial scuffles worth mentioning.

It’s also very easy to ignore the Corydoras’ need for space since the catfish maintains a low profile and doesn’t seem to require too much swimming room. I recommend providing your catfish with proper space even if you don’t think they need it.

Go for at least 10 gallons for 2-4 1-inch Corydoras. Larger species require more space.

7. Overfeeding

Overfeeding is a particularly dangerous problem due to stemming from people’s good intentions. We all want our fish to be well-fed and happy.

The problem is that the fish themselves don’t have an ‘I’ll stop now feeding button that they can press when full. It’s normal for fish to overeat, provided there’s sufficient food available. Such is the case with Corydoras, which are even more prone to this problem.

Corydoras already perform their natural scavenging activity, so they will get a lot of their sustenance from their environment. Especially when kept in a community setup where other fish provide them with plenty of food leftovers for them to enjoy.

Sure, Corydoras still require a balanced diet outside of what they can scavenge, but how much food they need varies based on the fish’s size, the environment, how many food residues there are available, etc.

Overfeeding is often linked to digestive problems like swim bladder disease and environmental issues due to excess fish waste and uneaten food leftovers.

So, your catfish can face numerous issues due to overfeeding alone.

Always understand your catfish’s nutritional needs and feed them accordingly, as opposed to adopting a standard feeding routine for all fish.

Corydoras need around 2 decent meals per day with sufficient food for them to consume in less than 2 minutes.

8. Temperature Shock

Temperature shock shouldn’t be a concern in a well-balanced environment with steady water temperatures. Most Corydoras require water temperatures between 74 and 80 F, and they appreciate stability above all else.

Temperature fluctuations can disturb the fish and even trigger stress responses. Temperature shock can occur in some conditions and can cause sudden death if severe enough.

Most catfish experience temperature shock due to the aquarists skipping the acclimation period necessary before placing the new-bought fish into the tank. The effect is the result of the fish exchanging different environments with drastically different temperatures.

To prevent this problem, always acclimate your catfish to their new living conditions gradually. You can do this simply by placing the bag containing the fish into the tank water, only partially submerged.

Keep the bag there for about 15-30 minutes, enough for the temperatures to equalize. Only then can you consider the temperature acclimation process complete.

Also, invest in a heater to prevent drastic temperature fluctuations between day and night.

The heater will maintain water temperatures stable and operate automatically to control environmental temperatures in case of unexpected fluctuations.

9. Low Oxygen Levels

Catfish are some of the most adaptable fish in this sense, but they’re not that adaptable. Corydoras are known as intestinal breathers.

This means that, just like labyrinth fish (bettas, among others), they can breathe atmospheric air in case of poor water oxygenation.

They achieve this by reaching the water’s surface and getting a fast gulp of air before diving back to their dwelling area.

This allows the fish to survive in low-oxygen waters, although you shouldn’t take this ability for granted. Intestinal breathing is a last-resort evolutionary feature that allows the fish to survive in extreme conditions, but you shouldn’t expect your catfish to thrive in such an environment.

Low-oxygen waters will eventually affect your catfish, causing visible signs of stress and hypoxia (asphyxiation.)

Always monitor water oxygen levels, add more live plants, and get some air stones if the situation really demands it. Also, provide your catfish with free access to the water surface if they need some occasional air gulps.

They won’t engage in the behavior too often if the water is well-oxygenated, but it doesn’t hurt to provide your catfish with more options in case of need.


Corydoras aren’t exactly sensitive and fragile, but they’re not impervious to diseases and health issues either.

Many things can go wrong, especially when not properly managing your catfish’s environment.

Follow my recommendations, understand your catfish’s needs and preferences, and keep an eye on their activity.

This should provide you with a robust baseline for creating a healthy, stable, and comfortable catfish setup at home.

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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