Cory Catfish Have Red Gills – Reasons & Solutions

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If you know anything about Corydoras, you know that they’re typically hardy and adaptable fish that don’t need much care and maintenance.

So, it’s always unnerving to discover your catfish’s red gills since this is almost always a sign of health problems.

Considering that some of these health problems are generally fatal, you need an immediate course of action to diagnose and address the condition.

So, let’s start with the beginning!

5 Reasons Why Cory Catfish Has Red Gills

The following are the 5 reasons why Corydoras showcase red gills, so let’s start with the worst one:

1. Ammonia Poisoning

This is a condition with an extreme death potential. Ammonia poisoning is a severe disorder that occurs when the fish comes into contact with environmental ammonia.

The symptoms will be mild at first but will progress fast, depending on the level of ammonia and the fish’s specific sensitivity.

The main symptoms of ammonia toxicity include red gills, duller body colors, increased mucus production, accelerated breathing, signs of suffocation, etc.

In many cases, the fish will also showcase erratic swimming, lack of appetite, lethargy, and a comatose-like state before death occurs.


Ammonia is the natural outcome of organic matter decaying in the water. This matter serves as food for various microorganisms, which use it as sustenance and produce ammonia in return.

Denitrifying bacteria consume ammonia regularly, but they may not suffice in all scenarios.

A combination of artificial filtration and regular tank maintenance is necessary to prevent the accumulation of ammonia in the water.

Possible Treatments

If your fish shows signs of ammonia toxicity, consider the following:

  • Quarantine the fish – This is the best course of action, as removing the fish from the toxic environment is key to its survival. This means having a hospital tank ready, preferably with similar parameters as the main aquarium. If there isn’t any, prepare a container in this sense. If that’s not an option, consider the following options.
  • Water changes – You must perform at least one water change daily (25-50% of the total volume) to dilute ammonia quickly. Make sure to use dechlorinated water and check the water’s temperature and nutrient content. The water should have similar characteristics to the tank water, minus the ammonia, of course.
  • Ensure optimal chemical filtration – If you don’t already have chemical filtration in your Corydoras tank, now’s the time to get it. You can use zeolite for your freshwater tank to trap harmful chemicals, ammonia being one of them. For the record, I recommend investing in a chemical filter to prevent this problem in the future.
  • Add salt – You need 1 to 2 grams of salt per liter of water to aid the fish’s kidneys.
  • Antibiotics – Antibiotics have no effect on ammonia, but they help with any secondary infections that are common with ammonia poisoning. These will speed up the fish’s recovery.
  • Stop the feeding – Your catfish will be fine without food for 24-48 hours. The goal is to stop the feeding for a while to minimize the amount of ammonia-producing organic matter. The lack of food will also cause the fish to poop less for the same effect. If your fish still have a healthy appetite, you can feed them but do your best to prevent any food leftovers along the way.
  • Improved water oxygenation – This is a must, given that ammonia poisoning causes the fish to experience difficulties breathing. Air stones can prevent this problem by improving oxygen levels, allowing fish to breathe better.

Moving forward, you can try to decrease the number of fish in the tank and adopt a strict maintenance routine to prevent dangerous ammonia buildup.

Also, if you have a sandy substrate, stir and vacuum the substrate occasionally to prevent the formation of anaerobic pockets.

2. Lack of Nutrition

Corydoras are often subjected to malnourishment, especially due to inexperienced aquarists misinterpreting the fish’s capabilities.

Corydoras rank as cleaning fish, so people assume they can survive on food scraps, they will collect from the substrate. This is false.

Corydoras also require a balanced diet with at least 2 meals per day. They need omnivorous meals with more animal protein and diverse amounts of veggies, depending on their liking.

You should provide your Corydoras with sufficient food for them to consume in 3 minutes or less.

Malnourished catfish can exhibit signs of nutrient deficiency, red gills being one of them.

3. Gill Flukes

Gill flukes are parasitic flatworms that begin their life cycle in the fish’s intestinal system. The resulting larvae attach themselves to the fish’s gills and start feeding on the fish’s blood.

This causes the gills to be flooded with blood which is responsible for the trademark redness in the area. You should be able to observe the larvae with the naked eye at a close inspection.


The parasite can simply enter the water via 3rd party means like infected plants or fish. It won’t take long for the organism to move to healthier hosts and infect all fish in the tank.

Immediate treatment is necessary to eliminate the pathogen. Poor water conditions and bad food are also some of the potential causes.

Possible Treatments

The treatment should consist of 3 phases:

  • Quarantine – We’ve already discussed the importance of quarantine, the primary benefit being the isolation of the sick specimen. This will contain the parasite’s spread and allow for improved control during the treatment.
  • Talk to a vet – Gill flukes are difficult to combat, especially in advanced stages. I recommend speaking to a vet to figure out the best line of attack. You want to eliminate the parasite without causing your fish too much discomfort in the process.
  • Salt and medication – A bit of salt added to the environment may affect the parasite, but this isn’t a failproof method. Drugs like fenbendazole, praziquantel, or levamisole hydrochloride can be especially effective in this sense, but be careful. These medications can be remedy or poison, depending on the dose. Always discuss the issue with your vet before using anything.

4. Aggressive Mates

This is another interesting problem that many aquarists tend to overlook. Corydoras are peaceful and timid fish that don’t like to get into scuffles with other tankmates.

They always prefer to flee and hide whenever attacked, bullied, or poked on by overly curious fish.

The problem arises when you overlook the catfish altogether, which is easy to do due to the fish’s low profile.

The catfish isn’t equipped to handle attacks and bullying from their tankmates, so they can easily sustain damage, especially in the areas not protected by their plates. The gills are the obvious target in this sense.

The catfish can also attempt to flee the area of attack in a hurry and hide among rocks, during which it can sustain incidental skin damage.

You should always avoid aggressive, large, or overly inquisitive tank companions that can bother the catfish and invade its space too often.

How to Prevent Red Gills on Cory Catfish?

It goes without saying that prevention remains preferable to treating the problem once it’s set in.

Here are some great prevention techniques to consider in this sense:

  • Prevent and control ammonia buildup – Ammonia is the natural result of decaying organic matter, but can also form as a result of anaerobic sand pockets. The latter is vastly more prevalent in aquariums with thick sand substrates. To prevent ammonia buildup, stir and vacuum the substrate regularly, have a regular water change routine, and clean the tank of food residues and dead organic matter as often as possible. Also, have a tester kit nearby to verify water parameters occasionally.
  • Ensure a proper dietCory catfish require a healthy and balanced meal plan that would mix animal protein with the occasional veggies for a plus of minerals and vitamins. Sure, Corydoras get much of their sustenance via scavenging, but this isn’t enough to satisfy their nutritional requirements. Fortunately, this species isn’t pretentious, and it will eat anything; with the mention that live food prevails.
  • Maintain high water quality – The goal is to prevent infections and parasites. The tank water can get dirty fast, turning the entire aquatic environment into a ticking bomb. A stable maintenance schedule should prevent any problems in the long run. Also, always quarantine new fish and sterilize any live plants, rocks, or decorations you plan on adding to the tank. These often serve as transportation means for hitchhiking parasites, worms, and snails that can infect the aquarium.
  • Find compatible tankmates – Corydoras are peaceful and timid, so their tankmates should follow the same formula. Only pair your catfish with equally peaceful and friendly fish and aquascape the fish’s habitat wisely. Add more live plants, rock structures, and various other decorative elements that the catfish can use as hiding and safe spaces.


While they are hardy and resilient, Corydoras also rank as vulnerable to improper water conditions.

Dirty and poorly oxygenated waters will cause the fish to experience discomfort and stress, which lower its immune system along the way. This opens the door to a myriad of problems.

You can solve all these issues easily by adopting a strict maintenance schedule, feeding your catfish properly, and sticking to a personalized care routine over the years.

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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