What is Red Pest Disease, And How Do You Treat It in Fish?

Written by Kirstin Harrington
Published: May 3, 2023
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Whenever someone gets a new pet, their goal is to give it a happy, healthy, and safe life. To do that, no matter what type of animal you have, it’s essential to pay extra close attention to their health. 

Today we’re going to be looking at a disease our finned friends can get called red pest disease. Red Pest Disease is among the most prevalent conditions that affect fish.  

Red pest disease can afflict fish of any color, but because it forms red lesions on the fish’s body, it can look particularly scary in goldfish.

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The causes, signs, and remedies of fish red pest disease will be covered in this article. You may take action to protect your fish from contracting this disease if you understand what to look for.

Lionhead goldfish died due to poor water quality i.e. ammonia poisoning. Dead Small fish on the surface of water.

Red pest disease can afflict fish of any color, but because it forms red lesions on the fish’s body, it can look particularly scary in goldfish.

©Zay Nyi Nyi/Shutterstock.com

What is Red Pest Disease?

Red pest disease, one of the most widespread fish illnesses, is a bacterial infection brought on by Bacterium cyprinid. The majority of ponds and aquariums contain this bacteria. The bacteria can only result in illness when the fish’s immune system is weak.

Although the illness is not communicable, it might gradually cause the fish life in your tank to decline. Fish with the infection may at first simply exhibit a few red sores on their bodies. However, if neglected, the illness can lead to mortality, organ damage, and fin rot.

The sickness in fish is frequently confused with Columnaris, another prevalent bacterial fish disease. Red sores on the fish’s body are the result of both diseases. Red pest illness, however, also affects the fish’s eyes, scales, and internal organs, in contrast to Columnaris, which mostly affects the fish’s fins, gills, and mouth.

Signs Your Fish Has Red Pest Disease

Now that you’re aware of what red pest disease is, let’s take a look at the signs and symptoms this disease creates. Knowing what to expect can keep you ahead of the game and possibly save your pet from misery. 

Kohaku Koi fish died due to poor water quality i.e. ammonia poisoning. Caught by fishing net. Right upper view.

Fatigue is another typical sign of red pest sickness.

©Zay Nyi Nyi/Shutterstock.com

Blood Marks on Their Body

Bloody spots or stripes on the fish’s body are one of the red pest disease’s most prevalent symptoms. These lesions, which can develop anywhere on a fish’s body, fins, scales, or tail, are often red or pink. These spots are frequently most noticeable on the fins of goldfish.

As soon as you notice these markings on your fish, you must act right away. The illness can spread and ultimately kill the fish if it is not addressed.  Not only that, but the lesions may also cause issues for additional fish in the tank, increasing their susceptibility to illness.

Loss of Appetite

Fatigue is another typical sign of red pest sickness. When this happens, the fish exhibits a vacant and apathetic demeanor. The fish might completely cease eating in some circumstances.

It’s crucial to investigate more closely if you find that the fish are not acting normally or are no longer eating. It’s wise to be extra cautious because tiredness and loss of appetite might be early indicators of sickness.

Additional Mucus on Scales

A small bit of mucus is produced by healthy fish to safeguard their outermost layer and scales. Yet, sick fish frequently create an excessive amount of mucus. As a result, the body of the fish can look sticky and slick.

You might observe that the fish’s fins seem constricted alongside a spike in mucus production. At this point, the fish fins are tightly clasped to their bodies. Fish with clamped fins may be under stress, which makes them more prone to illness.

Causes of Red Pest Disease

Let’s examine the causes in more detail now that we are more familiar with what red pest sickness is. Stress is the main factor in red pest disease, therefore anything that stresses out your fish might compromise their immune system thereby rendering them more vulnerable to the illness.

The following are typical factors that can result in red pest illness. 

Koi fish swim in the clear pond and are eating.Many orange and white koi fish in the pond are clear and clean.

It’s crucial to only put as many fish in your tank as it can safely hold.

©attakorn sanguanwong/Shutterstock.com

Too Many Fish in a Tank

Bad tank water conditions are frequently a consequence of overstocking. Fish excrement can quickly accumulate in a tank with too many fish, which can cause increases in ammonia and nitrite levels. The fish might experience extreme stress from this contaminated water, which increases their susceptibility to illness. 

It’s crucial to only put as many fish in your tank as it can safely hold. One inch of fish for each gallon of water is an adequate guideline to follow. Alternately, allot one liter of water for every fish that is two inches long.

Not only will this lessen the chance of dangerous diseases, but it will also enhance the life of your fish and help them thrive in their environment. 

Unhealthy Water Conditions

The water in a fish tank is the animal’s life force. It’s essential to keep it pristine at all times. One of the main factors that might promote red pest disease in fish is poor water sanitation.  High concentrations of ammonia, nitrites, and nitrates in the water can harm fish gills and impair their ability to breathe. 

As a result of this strain, the fish may become more prone to illness.

Monitor the water in your tank frequently and take precautions to keep ammonia, nitrite, and nitrate levels minimal to ensure the safety of your fish. You can employ a superior filtration system and do routine water replacements. 

Age of Fish

Tragically, as they become older, fish become more prone to illness. Their immune systems deteriorate with age, making them less able to fend off infections. Goldfish are more susceptible to many fish diseases and parasites since they are weaker fish.

One can take measures to ensure that the fish you own have a healthy and long life despite the fact there is no way to stop them from aging. The most important thing is to give them the finest care possible, which includes frequent water changes and nutritious feed. If given proper care, more fragile fish can enjoy a long life.

Lack of Nutrition

Fish require proper nutrition to keep healthy, just like any other living thing. A poor diet inadequate in nutrients such as vitamins and minerals can cause fish to have a variety of health issues, including a compromised immune system. 

They might be more prone to illness as a result. Give your fish premium fish food made especially for their species when possible. This helps to ensure that they are getting the nutrition they require. 

Treatment Available for Red Pest Disease

This type of disease can make the fish severely unwell and perhaps kill them if it is not addressed. One of the most typical side effects of this illness is fish septicemia, sometimes known as a blood infection. 

The fish will soon start to experience organ failure and eventually pass away. This is why it’s crucial to act as soon as you discover any of the aforementioned symptoms or indicators.

The positive aspect is that it is treatable to eradicate the fish’s red pest disease. In most cases, the most important thing is to identify it early and begin treatment right away. The sooner you begin treatment, the greater the likelihood that your fish will recover completely.

Coral reef saltwater aquarium tank

Natural disinfectants like aquarium salt can kill dangerous bacteria and parasites that enter the aquarium.



To treat an advanced form of red pest illness, antibiotics may be required. Tetracycline and chloromycetin are a couple of the most widely used antibiotics for treating fish infections, while other antibiotics can also be utilized. 

If you must administer antibiotics, make sure to follow your veterinarian’s instructions exactly. According to your veterinarian’s advice, you should typically provide antibiotics with fish food. It might also be necessary to put antibiotics into fish that are debilitated. 

This should only be carried out under the guidance of a licensed veterinarian since if done incorrectly, it can be quite harmful. Similar to humans, with any prescription drug for fish, it’s crucial to complete every dose of antibiotics, regardless of whether your fish appears to be doing better. 

Bacterial strains that are resistant to therapy can develop and are far more challenging to eradicate.

Utilize Aquarium Salt

Natural disinfectants like aquarium salt can kill dangerous bacteria and parasites that enter the aquarium.  Several common fish ailments, including red pest disease, can be effectively treated with it as well.

Be sure to use aquarium salt according to the package’s instructions. For every gallon of water, the majority of brands advise adding five tablespoons of non-iodized aquarium salt. By doing this, a fish bath will be made that can be utilized to heal sick fish.

Extended exposure to aquarium salt can harm fish that have not gotten used to it, which means you should only apply it sparingly. As soon as the treatment is complete, you have to take out all non-iodized salt from the water. 

Prescription Medication

Fish disorders can be treated with a wide range of external medicines. Methylene blue, an efficient disinfectant, is one of the best. Methylene blue must be used exactly as instructed otherwise it could be hazardous to fish. 

Malachite green is yet another typical drug used to address red pest disease. Although it may cause certain undesirable side effects, using it with fish is typically regarded as safe. It’s crucial to take out the carbon from the filter before utilizing external drugs, as is the circumstance with the majority of hospital tank treatments. 

This will guarantee that the drug can carry out its intended function and won’t cause the activated carbon to absorb it. After therapy, keep a close eye on your fish. If the illness doesn’t go away after the first round of treatment, you might need to try it again 

Keep a Clean Tank

It is essential to do routine water changes. The parameters of your water should already be routinely tested with a water testing kit. However, it becomes even more crucial to monitor water quality when your fish are sick. 

Toxins and other dangerous elements are removed from the water with the aid of water changes. The vital minerals and nutrients that fish require to remain healthy are also replenished. A minimum 50% water change should be performed for addressing red pest disease.

If you have a pond that contains a large number of fish, you might need to do several alterations to the water during the week. This will support preserving fish welfare by keeping the water clear and toxin-free.

Final Thoughts

Red pest illness is a dangerous condition that, if left untreated, can be deadly to fish.  Red pest disease can be treated with a variety of methods, including aquarium salt, water modifications, and external drugs. 

Antibiotics might be required in some situations as well. While receiving treatment, keep a close eye on your fish and get in touch with a licensed vet if the illness does not go away.

A to Z Animals trusts that this post has helped you comprehend red pest disease and its treatments better. Good luck to you and your little finned friends! 

The photo featured at the top of this post is © M-Production/Shutterstock.com

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About the Author

Kirstin is a writer at A-Z Animals primarily covering animals, news topics, fun places, and helpful tips. Kirstin has been writing on a variety of topics for over five years. She has her real estate license, along with an associates degree in another field. A resident of Minnesota, Kirstin treats her two cats (Spook and Finlay) like the children they are. She never misses an opportunity to explore a thrift store with a coffee in hand, especially if it’s a cold autumn day!

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