Goldfish Red Pest Disease – Causes, Symptoms And Treatments

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Goldfish make remarkable pets. They are low-maintenance, long-lived, and can even be trained to do tricks! But like all living creatures, they are susceptible to disease. One of the most common goldfish diseases is Goldfish Red Pest Disease. Though it can affect fish of all colors, red pest disease in goldfish can appear particularly alarming due to the red lesions it causes on the fish’s body.

This article will discuss the causes, symptoms, and treatments of goldfish red pest disease. Once you know what to look for, you can take steps to prevent this disease from affecting your fish.

What Is Goldfish Red Pest Disease?

One of the most common fish diseases, red pest disease is a bacterial infection caused by Bacterium cyprinid. This bacteria is found in nearly all ponds and aquariums. Only when the fish’s immune system is compromised can the bacteria cause disease.

Though the disease isn’t contagious, it can slowly lead to the deterioration of fish life in your tank. Infected goldfish may initially only show a few red lesions on their bodies. But if left untreated, the disease can cause organ damage, fin rot, and death.

The disease in fish is often confused with another common fish disease caused by bacteria, Columnaris. Both diseases cause red lesions on the fish’s body. But while Columnaris primarily affects the fins, gills, and mouth of the fish, red pest disease will also affect the eyes, scales, and internal organs.

What Causes Goldfish Red Pest Disease?

ranchu with red spots
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Now that we know what red pest disease is, let’s take a closer look at the causes. The root cause of red pest disease is stress, so anything that causes stress to your fish can weaken their immune system and make them more susceptible to the disease.

Common stressors that can lead to red pest disease include:

Poor Water Conditions

Poor water quality is one of the leading stressors that can cause red pest disease in goldfish. Ammonia, nitrites, and nitrates present in high levels in the water can damage the fish’s gills and make it difficult for them to breathe. This stress can weaken the immune system and make the fish more susceptible to disease.

To keep your fish safe, it’s important to regularly test your water quality and take steps to maintain the levels of ammonia, nitrites, and nitrates low. You can perform regular water changes and use a high-quality filtration system.


It is often the case that poor tank water conditions are the result of overstocking. When there are too many fish in a tank, their waste can quickly build up and lead to ammonia and nitrite spikes. This dirty water can cause immense stress to the fish and make them more susceptible to disease.

Ranchu goldfish

It’s important to only stock your tank with as many fish as it can safely accommodate. A good rule of thumb is to only allow for one inch of fish per gallon of water. Alternatively, allocate a liter of water for each two-inch fish.

Poor Nutrition

Like all living creatures, fish need a balanced diet to stay healthy. A low-quality diet deficient in vitamins and minerals can lead to a number of health problems in fish, including a weakened immune system. This can make them more susceptible to disease.

To ensure your fish are getting the nutrients they need, feed them a high-quality fish food formulated specifically for goldfish. You can also supplement their diet with live or frozen foods like brine shrimp, daphnia, and bloodworms.


Unfortunately, goldfish are more susceptible to disease as they age. As they get older, their immune system weakens and becomes less able to fight off infection. By virtue of being weaker fish, older goldfish are more likely to fall prey to various fish parasites and diseases.

While there’s no way to prevent your goldfish from aging, you can take steps to make sure they live a long and healthy life. The key is to provide them with the best possible care, including regular water changes and a high-quality diet. Even weaker goldfish can live long if they’re well cared for.

Signs and Symptoms of Red Pest Disease

Now that we know what red pest disease is and what causes it, let’s take a look at some of its key signs and symptoms:

Bloody Streaks and Patches on Body

One of the most common signs of red pest disease is bloody streaks or patches on the fish’s body. These lesions are usually red or pink and can appear anywhere on the body, fins, scales, and tails of fish. In goldfish, these lesions are often most visible on the fins.

Once you spot red spots on goldfish, it’s important to take action quickly. If left untreated, the disease can spread and eventually kill the fish. And that’s not all – the lesions can also be a source of stress for other fish in the tank, making them more susceptible to disease.

Increased Mucus Production

Healthy fishes produce a small amount of mucus to protect their skin and scales. However, when fish are ill, they often produce mucus in excess. This can cause the fish’s body to appear slimy and slippery, similar to what you would notice with goldfish slime disease.

In addition to an increased mucus production, you may also notice that the fish’s fins appear clamped. This is when the fish fins are held close to their bodies tight. Clamped fins can be a sign of stress, making fish more susceptible to disease.

Lethargy and Loss of Appetite

Goldfish eating

Another common symptom of red pest disease is lethargy. This is when the fish appears sluggish and uninterested in its surroundings. In some cases, the fish may stop eating altogether.

If you notice that your goldfish are not as active as usual or have stopped eating, it’s important to take a closer look. Lethargy and loss of appetite can be early signs of disease, so it’s best to err on the side of caution.

Treating Goldfish Red Pest Disease

If left untreated, pond pest disease may cause your goldfish to become seriously ill and can even result in dead fish. Fish septicemia, or a blood infection, is one of the most common complications of this disease. It is only a matter of time before the fish’s organs begin to fail and eventually die. This is why it’s so important to take action as soon as you notice any of the above signs and symptoms.

The good news is that goldfish red pest disease in goldfish can be treated. The key is to catch it early and start treatment immediately in most cases. The earlier you start treatment, the better the chances are of your fish making a full recovery.

Perform A Water Change

One of the most important aspects of fish care is performing regular water changes. You should already be testing water parameters with a water testing kit on a regular basis. But when your fish is sick, it’s even more important to keep a close eye on water quality.

Young man changing water in aquarium using siphon.

Water changes help to remove toxins and other harmful substances from the water. They also replenish essential minerals and nutrients that fish need to stay healthy. When treating red pest disease, it’s important to perform a water change of at least 50%.

If you have a lot of fish in your pond, you may need to perform multiple water changes throughout the week. This will help keep the water clean and free of toxins, thus maintaining the welfare of fish.

Aquarium Salt

In addition to performing regular water changes, you should also add aquarium salt to the water. Aquarium salt is a natural disinfectant and can help to kill harmful bacteria and external parasites. It’s also an effective treatment for many common fish diseases, including red pest disease.

When using aquarium salt, be sure to follow the directions on the package. Most products will recommend adding 5 teaspoons of non-iodized aquarium salt per gallon of water. This will create a fish bath, which can be used to treat sick fish.

You should only use aquarium salt for short periods of time, as long-term exposure can damage the body of fish that are not used to it. Be sure to remove the non-iodized salt from the water once the treatment is finished.

External Medications

methylene blue

There are a variety of external medications that can be used to treat fish diseases. One of the most effective is methylene blue, a powerful disinfectant. It’s important to use methylene blue as directed, as it can be toxic to fish if not used properly. Another common medication used to treat red pest disease is malachite green. Though it can produce some unwanted side effects, it’s generally considered safe for use with goldfish.

As is the case with most hospital tank treatments, it’s important to remove carbon from the filter before using external medications. This will ensure that the medication can do its job and doesn’t get absorbed by the activated carbon.

After treatment, be sure to monitor your fish closely. You may need to repeat the treatment in some cases if the disease does not clear up the first time.


Last but not least, antibiotics may be necessary to clear up a severe case of red pest disease. A variety of antibiotics can be used to treat fish diseases, though tetracycline and chloromycetin are two of the most common. If you need to use antibiotics, it’s important to do so as directed by a qualified veterinarian. Generally speaking, you will serve antibiotics with fish food according to your vet’s instructions.

Severely weakened fish may also need to be given injections of antibiotics. This should only be done under the supervision of a qualified vet, as it can be very dangerous if not done correctly.

As with all medications, it’s important to finish the entire course of antibiotics even if your fish seems to be feeling better. Incomplete treatment can lead to drug-resistant strains of bacteria, which are much more difficult to treat.

The Takeaway

Red pest disease is a serious condition that can be fatal to goldfish if not treated properly. Some of the most effective treatments for red pest disease include water changes, aquarium salt, and external medications. In some cases, antibiotics may also be necessary. Monitor your fish closely during treatment and contact a qualified veterinarian if the disease does not clear up.

We hope this article has been helpful in better understanding red pest disease and how to treat it. As always, feel free to contact us with any further questions. And if you found this article helpful, be sure to check out our other articles on goldfish care!

Wanda is a second-generation aquarist from the sunny tropics of Malaysia. She has been helping her father with his freshwater tanks since she was a toddler, and has fallen in love with the hobby ever since. A perpetual nomad, Wanda does her best to integrate fish-keeping with her lifestyle, and has taken care of fish in three different continents. She loves how it provides a nice break from the hustle and bustle of life.

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