Why Are My Corydoras Dying (10 reasons with solutions)?

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I am a huge fan of Corydoras Catfish. I have been keeping and breeding many species of Corydoras for over 20 years. I have bred and sold Panda Corydoras, Pygmy Corydoras, and Peppered Corydoras to name just three. They are one of the fish I recommend breeding for profit.

Corydoras make a great addition to any aquarium thanks not only to their cheeky personality but also their ability to act as a clean-up crew, eating any food that lands on the bottom of the aquarium.

It can however be incredibly frustrating when our Corydoras appear to be dying for no reason. In this article, I go through the 10 most likely reasons your Corydoras Catfish are dying and how we can save them.

In my experience of keeping these beautiful fish, the following reasons are the most likely causes of Corydoras Catfish dying.

  • Aquarium Cycling Issues
  • Ammonia Spike
  • Poor Water Quality
  • Overfeeding
  • Pests & Disease
  • Stress
  • Low Oxygen Levels
  • Water Temperature
  • Poor Quality Diet
  • Overstocking

1. Aquarium Cycling Issues

When we set up an aquarium for the first time, we need to allow time for the bacteria which colonize our filters, to develop. These bacteria, which are often referred to as beneficial bacteria, process the waste from our fish, keeping the water clean and safe for the fish.

The process of allowing an aquarium to go from freshly filled with tap water to ready to receive fish is known as cycling the aquarium.

It can take a couple of weeks for sufficient bacteria to develop, depending on stocking levels, and if we add too many fish too quickly, the bacteria struggle to multiply quickly enough to deal with the waste from the fish.

Some local fish stores will say things like ‘just leave your aquarium for 24 hours before adding fish’ or worse still they will sell you fish the day you buy the new tank from them.

When the amount of waste produced by the fish outweighs the amount of waste the available bacteria can process, ammonia levels can increase to the point where our Corydoras Catfish start to die. This is known as having cycling issues.

The solution to cycling issues is to carry out one or more large partial water changes. By draining 25% to 50% of the tank water out and replacing it with fresh, dechlorinated tap water, the amount of ammonia present in the water with be reduced by 25% to 50% immediately.

Carrying out a water change, or two or three water changes over a week or two will give the bacteria sufficient time to reproduce.

2. Ammonia Spike

As mentioned above, when our Corydoras Catfish go to the bathroom, their waste is very high in ammonia, and ammonia is toxic to fish. Ammonia can kill our Corydoras Catfish, even when it is only present at relatively low levels. Our aim should be to have no ammonia present in the aquarium, ever.

It is the job of the bacteria that live in our filters (often referred to as beneficial bacteria) to convert this toxic ammonia into the less toxic nitrate. This process is known as the Nitrogen Cycle.

Exactly as the name suggests, an ammonia spike is when the level of ammonia present in the water increases suddenly for some reason. Ammonia spikes can happen quickly and without warning.

The most common causes of an ammonia spike are either overfeeding the fish (see more below), or when a fish dies, and we fishkeepers do not notice. When dead fish decay in the aquarium water, they give off large amounts of ammonia.

Sometimes, an ammonia spike may be caused by a power failure. As mentioned, our filters are the place the majority of the beneficial bacteria can be found, and that bacteria requires a constant supply of oxygen-rich water to survive.

When the is a power failure, the water typically stops flowing through the filter, which can cause the bacteria to die. If the power failure lasts for a couple of hours or more, when the power returns and the filter starts running again, not only will there be little to no bacteria left alive to process the fish waste, but the dead bacteria themselves will become a source of ammonia.

If you believe you are dealing with an ammonia spike, the most effective course of action is to carry out an immediate, large partial water change. Changing 50% to 75% of the aquarium water will have a huge impact on the ammonia levels in the water.

This water change may need to be repeated several times over the coming week or two whilst the bacteria levels rebuild.

To check if you are suffering from an ammonia spike, you will need to test your aquarium water. See more about testing in the section below.

3. Poor Water Quality

I have a feeling poor water quality kills more Corydoras Catfish than all the other reasons put together. The biggest problem with water quality is you can’t tell the quality just by looking at it. Water can be very poor chemically, but crystal clear to look at.

I have given countless fishkeeping talks, and in the Q&A sessions at the end, someone will frequently ask me why their fish are dying. When I ask them what the results of their testing were, they will reply with something like ‘oh, I haven’t tested, but my water is crystal clear, so the quality isn’t the problem!’.

When we say the water quality is poor, we are generally referring to the levels of ammonia, nitrite, and nitrate present in the water.

As discussed above, when our fish go to the bathroom, their waste is high in ammonia. The bacteria that live in our filters convert that ammonia into nitrite. Nitrite is less toxic than ammonia but still fairly toxic to our fish.

Fortunately, there is a second strain of bacteria that lives in our filters that convert the nitrite into the far less toxic, nitrate. Fish can tolerate fairly high levels of nitrate in their water.

This process is known as the Nitrogen Cycle.

Many people will incorrectly suggest your aquarium must have 0ppm (parts per million) of nitrate present in the water. On many forums and fishkeeping websites, the number 40ppm is often banded around and suggested as a maximum level of nitrates that should be present.

In my experience, many fish, including Corydoras Catfish can easily cope with nitrate levels of 100ppm or more.

There are two ways we can lower nitrate levels in our aquariums. The first is to plant live aquarium plants. Live plants absorb nutrients from the water column as they grow, and these nutrients include nitrates. Generally speaking, aquariums with live aquatic plants growing have lower levels of nitrates than those that do not.

The second way is to carry out regular partial water changes. In my own fish room, I generally change between 10% and 25% of the water in each tank once a week.

How Do You Know Your Aquarium Water Quality Is Poor?

As mentioned above, you can not tell the quality of your aquarium water just by looking at it. Neither ammonia, nitrite nor nitrate has any color to them. Crystal clear water can be very poor chemically.

The only way we can judge the quality of our water is by using an aquarium test kit. Test kits give us an almost instant result, allowing us to take the appropriate action.

There are two ways we can test our aquarium water, either with a liquid test kit or by using test strips. I have spent some time reviewing which aquarium tests are the best, and you can find out more in this article titled Best Aquarium Test Kits (tried and tested).

Most test strips and liquid test kits will test for pH, ammonia, nitrite, nitrate, and chlorine, and many will test even more parameters.

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Test strips are quicker to use than liquid test kits. Test strips will often give you a result within around 60 seconds, whereas liquid test kits can take 5 minutes or more. However, in my experience, liquid test kits do tend to be more accurate.

How to fix poor quality water?

If you test your aquarium water and discover the quality is not as good as you had hoped, you will need to take immediate action to prevent further Corydoras Catfish from dying.

The best way to fix poor water quality is to carry out an immediate partial water change. When I find one of my tanks has poor water quality I typically carry out a 25% to 50% water change, replacing the existing tank water with fresh, dechlorinated tap water. I will usually then re-test the water after 4 or 5 days, and repeat the large water change if necessary.

The quicker you act with the water change, the fewer fish are likely to die as a result of the poor water quality.

4. Overfeeding

Overfeeding is another really common way we can end up killing our Corydoras Catfish. Personally, I find feeding my fish to be the most enjoyable part of the hobby. I don’t find it a chore, I enjoy watching my fish swimming around gobbling the food up.

Where many fish keepers go wrong is they feed their fish 1 large meal once a day. Typically, people put a single large pinch of food into the aquarium, and much of that food gets past the fish before they have a chance to eat it. Uneaten food then ends up landing in clumps of plants or behind decorations.

Whilst Corydoras Catfish are famed for their abilities to clear up uneaten food, there will be places in the aquarium even they cannot reach. Uneaten food that lands in inaccessible areas will rot, and rotting food releases ammonia into the water (see above).

Overfeeding is even more of a problem when we go on vacation and ask friends and family to feed our fish. Non-fishkeepers always look at the amount of food you have asked them to add and think that isn’t enough and they promptly add more food, exacerbating the problem.

In my experience, it is better to feed the fish 2 or 3 small meals spread out across the day than it is to give them one large meal once a day. Feeding little and often is better for the fish, better for the filter, and better for the aquarium water in general.

5. Pests & Diseases

Another major reason Corydoras Catfish might start to die is due to pests and diseases. If the Corydoras are new to us, there is a possibility they were already either infected by a disease or carrying a parasite.

If the Corydoras have been in the aquarium for a while, but we have introduced some new fish to the tank, there is a chance those new fish brought a disease or parasite into the aquarium with them.

In my experience, the four most common pests and diseases that affect our Corydoras Catfish are;

Clearly, there are more than just these 4 diseases our Corydoras can suffer from, but these are the ones we tend to see most often.

6. Stress

Stress is often underappreciated in the hobby. There are many factors that can cause our fish to become stressed.

Loud noises from outside the tank, children continually banging on the glass or even poor water quality can all cause our Corydoras Catfish to become stressed. Even being housed with larger, more boisterous fish can cause our Corydoras to become sufficiently stressed that their health is affected.

Stress in fish can be incredibly hard to diagnose. If you believe stress could be a cause of your fish dying, take some time to sit and observe the tank. Make changes as appropriate and monitor the fish over the coming days.

7. Low Oxygen Levels

Many people are surprised to learn that fish actually breathe oxygen. Unlike us, however, fish extract the oxygen directly from the water by passing water through their gills. They essentially draw oxygen from the water and exhale carbon dioxide back into the water.

Occasionally, dissolved oxygen levels in the water can drop so low the Corydoras can struggle to breathe.

Corydoras species as a whole have adapted to survive in water with lower oxygen levels. They can sometimes be seen swimming rapidly to the surface before diving back down to the bottom of the tank. This is a technique known as aerial respiration, and whilst it does not necessarily indicate your aquarium has low oxygen levels, it can be one indicator.

The best short-term solution to low oxygen levels in the aquarium is to carry out a large, partial water change. Draining out 25% to 50% of the aquarium water and replacing it with fresh, dechlorinated tap water will bring in plenty of oxygen-rich water for the Corydoras.

In the long run, the best solution to low oxygen levels is two-fold. Firstly, adding live aquarium plants will increase the amount of oxygen available to the fish. Just like plants on land, aquatic plants take in carbon dioxide from the water and give out oxygen, increasing the amount of oxygen available in the water for the fish.

Secondly, adding an airstone connected to an aquarium air pump will raise the oxygen levels. As the bubbles rise from the air stone, they agitate the water surface. This water movement allows an exchange of carbon dioxide in the water for oxygen in the air above. This is why fast-moving rivers typically have more dissolved oxygen in the water than slow or non-moving ponds.

In my own fish room, almost every single tank has an airstone bubbling away no matter what fish are living in the tank.

8. Water Temperature

Corydoras are tropical fish. They generally inhabit naturally warm waters and as such they want their aquarium water to be somewhere 72°F and 82°F (22°C and 27.5°C). With that said, some species of Corydoras, such as the Sterbai Corydoras can tolerate much warmer conditions. I keep a large group in my Discus tank which runs closer to 86°F (30°C).

Whilst Corydoras Catfish can tolerate temperatures both slightly above and slightly below these numbers for a short period, if they are kept for too long in an aquarium that is much colder than 72°F or much warmer than 82°F their health can quickly suffer and they may start to die (see stress above).

Fitting a good quality digital thermometer to the front of the aquarium is the best way to monitor the water temperature. I find that with a digital thermometer it is easy to check the temperature at a glance, even when you are just passing by the tank.

If you discover the water is too cold or too warm, take immediate action to prevent any further fish losses.

9. Poor Quality Diet

Corydoras Catfish are omnivores. This means they need a diet that is based on both meaty foods and plant matter. Corydoras are relatively easy to fish to feed as they spend almost their entire day rooting through the substrate looking for things to eat.

The trouble with Corydiras Catfish is they are often sold as a clean-up crew. Whilst it is true that they are an excellent fish for eating all that food that makes its way past your main display fish, they can’t survive only on scraps. Corydoras Catfish need to be fed a balanced diet just like all other fish.

For my own Corydoras Catfish, I like to feed a fast sinking food. Something that will quickly pass the other fish, otherwise the majority of the food gets eaten on the way down leaving nothing for the Corydoras Catfish.

I have had great success using Repashy Gel Food. It is a little more expensive than some other foods, but the ingredients are really high quality and my Corydoras Catfish thrive on it. It is especially good if you are looking to breed your Corydoras and sell them for a profit.

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10. Overstocking

If there is one major mistake we all make, including me, it is overstocking our aquariums. We keep a far higher number of fish compared to the volume of water than would occur anywhere in nature.

The problem with overstocking is it can cause almost all of the other 9 points on this list to kill your Corydoras. Overstocking certainly can lead to ammonia spikes, stress, diseases, and bullying.

If you are losing Corydoras Catfish due to overstocking, consider reducing the number of fish you have in the aquarium, or upgrading to a larger tank.

Adding additional filtration will certainly help a little when it comes to keeping an overstocked tank clean, as will increasing the frequency of the partial water changes you carry out.

How to Keep Corydoras Catfish Healthy

In the article above we have looked at some of the reasons, Corydoras Catfish can die unexpectedly. Below I look at ways we can help ensure our Corydoras Catfish remain healthy.

  • Regular water changes
  • Maintaining the aquarium
  • Disease prevention and treatment
  • Feeding a balanced diet

Regular Water Changes

As discussed above, fish have no choice but to live in the water we provide them. If water conditions become less than perfect they don’t have the option to just swim slightly further downstream.

Carrying our regular, partial water changes is in my experience the single best thing we can do to keep our Corydoras Catfish happy and healthy. I try to carry out weekly water changes of about 25% of each of my aquariums. This reduces nitrates in the water and keeps the water fresh.

Regular water changes allow us to eliminate an issue before it becomes a problem.

Maintaining the aquarium

Along with regular water changes, keeping up with your general aquarium maintenance will go a long way to stopping your Corydoras Catfish from dying. Cleaning your filters, vacuuming the gravel, and trimming plants are all jobs that help keep an aquarium system running and your fish thriving.

Disease prevention and treatment

Quarantining new fish gives us a chance to spot if they are suffering from any pests or diseases before we mix them with our healthy fish.

Disease prevention is always better than cure. Before adding any new fish to your aquarium, consider quarantining your new fish in a separate aquarium for at least one week before moving them into your main display tank.

Feeding a balanced diet

As mentioned above, Corydoras Catfish are omnivores. They need a wide variety of foods. For the biggest, strongest, healthiest fish, you need to feed them a balanced diet.

A good mix of dried foods, live foods, and frozen foods will help give your Corydoras Catfish all the vitamins and minerals they need to be active and colorful.

In Conclusion

Corydoras Catfish are generally speaking hardy fish that should live for 5 to 7 years when kept in good conditions. Many species of Corydoras Catfish will even breed in the community aquarium when conditions are right.

If you find your Corydoras Catfish are dying unexpectedly, work through the list above and try and address any issue you find. The sooner you start addressing the issue, the fewer fish you will lose.

If you are interested in learning more about Corydoras Catfish, why not check out my article about Pygmy Corydoras?

About the Author

I’ve been keeping, breeding, and showing tropical fish for nearly 30 years. Over that time I’ve done it all! I’ve had great success and I’ve made some really foolish mistakes (like the time I bought an Asain Walking Catfish). Read more…
Richard James

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