17 Most Common Fish Diseases and How to Cure Them

17 Most Common Fish Diseases and How to Cure Them

We all know how disheartening it is to see our beloved pets getting sick or restless. If you encounter your fish are sick and something is not right with them, the probable cause could be a parasitic, fungal, bacterial, or viral infection.

Out of so many reasons, the most common cause for the fish getting sick is due to parasites. Fish may have both internal and external parasites that cause them to have multiple issues. So, here we are with the 17 most common fish diseases that are encountered by freshwater fish. Check them out and find ways how to cure them.

  1. Anchor Worms

If you have a pond, the chances of your fish getting troubled with anchor worms are very high. They are a large parasitic crustacean from the Lernaea species that make fish their habitat by attaching to their skin. Anchor worms also bury their head on the fish’s muscles and make them sick. If you take a closer look, you can see that the bodies of the anchor worms are sticking out of the fish. If there are whitish-green thread-like structures coming out from the fish, they could be anchor worms.


  • Red and inflamed scales are the most common symptom.
  • Itchy skin due to which the fish are found rubbing their skin on the walls of the tank

How to cure

If the worms are visible, take forceps and carefully remove them by pulling on the body of the parasite. Once removed, apply topical antibiotic ointment to the scales to prevent any kind of bacterial infection. Later, you must clean the entire tank to get rid of eggs, larvae, or other anchor worms present there.

  1. Bacterial infections

Bacterial infections frequently occur among the fish, and the major reason behind it is an injury. When in a tank, fish often get injuries due to different reasons. While fighting with the other tank mates is the most common cause, an injury may also occur while handling the fish, fin-nipping, unsuccessful predation, and various types of collisions and abrasions. The most common bacteria that bother a fish is Aeromonas salmonicida.

Apart from injury, poor water quality, poor diet, and genetic traits also contribute to getting bacterial infections. Such infections lower the immunity of the fish, make them stressful, and easily allow other bacterial infections to set it.


  • Swollen abdomen
  • Red spots on the body
  • Ulcers on the gills
  • Enlarged eyes

How to cure

If the fish is infected with a bacterial infection, antibiotics are prescribed based on the type of bacteria. You can easily find different types of antibiotic medications at the fish store or from a vet. Generally, the solution of the prescribed antibiotic is put into the water. You may require transferring your fish into the quarantine tank while they are being treated. Also, it is important to clean the main tank thoroughly to remove all the bacteria and make it healthy for the fish.

  1. Constipation 

Difficulty in pooping can cause constipation, which in turn will make the fish very ill. Just like humans, the major reason that may cause constipation is improper diet. When the fish is not taking healthy food, eating abruptly, and with no proper schedule, it may cause constipation. Another reason for constipation is a parasitic infection. All these cause difficulty in fish to pass the feces properly.


When the fish is suffering from constipation, you will notice a bloated abdomen. Swim bladder disease also occurs when the fish are constipated. Here, you will see the signs of buoyancy issues.

How to cure

Just the ways we treat our constipation, you may treat fish too. Increasing the amount of fiber-rich diet is one of the best ways to help fish pass feces easily. You may need to deworm your fish, too, to keep the chances of parasites at bay. Another proven way to treat fish constipation is by adding Epson salt in the talk. Epson salt acts as a muscle relaxant that helps fish to poop easily. It is advised to add 1 to 3 teaspoons of salt per every 5 gallons of water.

  1. Carbon dioxide Poisoning 

If your tank has plants, they will emit carbon dioxide. The threshold level of carbon dioxide in a tank is 30 ppm above, which may get hazardous to the fish’s health. If you already have a planted tank yet injecting carbon dioxide, the chances of CO2 poisoning rise. CO2 poisoning may also occur when-

  • CO2 reactor is not functioning properly
  • Plants are not absorbing CO2 as the lights are not working properly


  • Fish breathing more rapidly
  • Fish gasping for air
  • Fish spending more time near the surface of the tank

How to cure

The two best ways to get rid of CO2 poisoning are-

  • Using an air stone to agitate the surface that causes carbon dioxide to dissipate from the water.
  • Adjusting the rate of carbon dioxide injection in the aquarium.
  1. Flukes 

Flukes are a class of parasitic worms that mostly affects the gills of the fish. Gill flukes are specific for fish species; for instance, they are more common among koi fish. Dactylogyrus and Gyrodactylus are the two most common flukes that are found on fish.

Generally, the flukes enter the tank from a newly bought fish. It is transferred from one tank to another through water or infected fish.


  • Rapid respiratory movements
  • Red skin
  • Gills moving rapidly
  • Chewed gills
  • Scratching against objects by the affected fish
  • A layer of mucus covering gills or body

How to cure

The flukes attach to the gills that, after removal through medications, may cause wounds. Such sites welcome secondary bacterial infections. Make sure that you apply antibiotics to treat the wounds that are left after fluke’s treatment.

  1. Clamped Fins

The condition when fins clamp close to the body of the fish is called clamped fins. There could be different reasons causing clamped fins. Firstly, when the water parameters are not right, the fish clamp fins close to their body. Another reason for this condition is the presence of external parasites.


  • Fish gets lethargic
  • Holds fins closer to the body even while swimming

How to cure

The best way to get rid of clamped fins is by checking the water parameters of the tank. If you find anything- temperature, pH, salinity, nutritional levels of the water out of the limits, make sure to maintain it as soon as possible. Later, you may check the fish for the presence of any external parasite. If there are parasites, try to remove them manually and apply antibiotics.

  1. Dropsy

When the fluid builds up inside the body cavity or tissues of a fish, it causes the disease called dropsy. Dropsy indicates a number of underlying issues among fish, making it a symptom rather than a disease. Polycystic kidney issues, liver dysfunction, bacterial or parasitic infections are some of the reasons that may cause dropsy. Stress also contributes to dropsy to a large extent.


  • Slightly swollen belly
  • Swollen abdomen
  • Fish scales stick straight out
  • Fish get pinecone shape

How to cure

The first step you need to do when the fish gets dropsy is transferring it to a quarantine tank. It will reduce the stress that the fish may be experiencing in the display tank. Once you transfer the fish, the next step is to check the water parameters of the tank. Check if the water quality is poor, there is the presence of parasites, overcrowding, or anything else. Fix the probable issue and make sure that the fish do not feel stressed anymore.

Dropsy may get fatal; therefore, timely medication is essential. Talk to your vet or the pet store owner to know the right medications and the dosage to help relieve the fish from this deadly disease.

  1. Freshwater Ich

The causal organism for freshwater ich is Ichthyobodo which is a protozoan parasite. Generally, ich infects the fish that are stressed due to poor water conditions, certain illnesses, overcrowding, etc. When the immune system of the captive fish is not working well, and they are getting stressed due to different reasons, ich develops. As it is a well-known fact that when the fish is under stress, it lowers the immune response; it is when the ich takes over and infects the fish.


  • It affects the gills and skin of the fish
  • Fish develops grey color to their skin
  • Fish gets lethargic, weak and do not want to eat
  • Spend more time near the top of the tank and gulping air
  • Rubbing or scratching their side on the tank

How to cure

One of the most common ways of preventing freshwater ich is using medications like Epson salt and potassium permanganate or copper sulfate in your tank’s water. Here, it is essential to take the advice of an experienced hobbyist or fish vet who will provide the best guidance based on their experience and knowledge.

  1. Freshwater Velvet

Freshwater velvet is caused by the parasite Oödinium pillularis or Oödinium limneticum. This is one of those diseases which is highly deadly and can easily wipe out every precious creature you have in your tank.

Although freshwater velvet may be present in many aquariums, they cause issues only when the fish are under stress, sick, or living in a tank with poor water quality. Also, the chances of getting the infection increase when they are being transported or experience sudden temperature fluctuations. All this causes them to have a lower immune response and greater chances of getting infections.


  • Fish getting lethargic
  • Not eating well
  • Weight loss
  • Clamps the fins closer to the body
  • Scratching the body against the surfaces of the tank
  • Skin pealing
  • Rapid breathing
  • Yellow to rust color making your fish appear velvet

How to cure

The few things that can help in treating freshwater velvet issues include-

  • Increasing the water of the tank by just a few degrees
  • Dim the lights of the aquarium for a few days
  • Treat the quarantine tank’s water with copper sulfate for at least ten days.

You can easily prevent your stock from getting freshwater velvet by keeping the newly bought fish in a quarantine tank prior to the display tank. Make sure that you provide proper water parameters and provide your fish with a proper diet to overcome all the chances of getting freshwater velvet and protect your precious pets.

  1. Fungus 

The two most common fungal infections observed among freshwater aquariums are Saprolegnia2 and Ichthyophonus hoferi. When the water in the tank is dirty and comprises dead and decaying organic materials, the chances of fungus development increase.

While most fungal infections harm the external organs of the body, a few may harm the internal organs and make the fish really sick.


  • White cottony or “furry” growths on fish
  • Grey cotton-like growth on the skin, gills, and fins
  • Affect the areas around the eyes

How to cure

The most effective way of treating the fungal infection is by cleaning the tank completely. Later, you can put potassium permanganate in your tank’s water. Make sure that you have removed all the external pathogens from the fish’s skin. You can check out the multiple fungal treatment options available at the pet store or online. Using branded products will always give the best results. Another way of treating the fungus is increasing the water temperature to 82 degrees because most fungal infections thrive in colder temperatures.

  1. Gill Mites

Gills are the most important part of a fish’s body. Gill Mites, just as the name indicates, attack the gills. These parasites attach to the fish’s skin and feed on their blood. When a new fish that is already infected with gill mites introduced in your tank, it causes the infection to the entire stock. It is the major reason why always quarantining the new fish before adding it to the main tank is a sensible idea.


  • Gills do not function properly
  • Difficulty in closing the gills fully
  • Fish spending most of the time on the surface of the tank to gasp air

How to cure

If you think that your fish is stressful and finding it difficult to swim, the reason could be gill mites. Check with your vet or talk to the pet store owner. When the disease is confirmed, use the right medications under the guidance of experienced aquarists or the vet for the best results.

You have to change the entire water of the tank to make sure no gill mites are there, and the water is in the best condition for your stock.

  1. Hole in the Head

Hole in the head is caused by a protozoan called Hexamita. If you see small indentions on the head and along the lateral line of the fish, the chances are it Hole in the Head disease, which happens due to the protozoa Hexamita.

This disease is common in Discus fish. Apart from the protozoa, Hole in the Head also occurs when there is a mineral deficiency in water or when activated carbon is used excessively and abruptly in the tank.


  • Small indentions appear on the skin
  • You will see slight depressions advancing into holes
  • Fish feel tiredness and sometimes stop eating

How to cure

The most effective way of treating Hole in the Head disease is in the quarantine tank and treating with API General Cure.

  1. Viral Hemorrhagic septicemia

Viral Hemorrhagic septicemia is caused by Piscine novirhabdovirus, which is a deadly virus and can affect the entire stock. It is a serious, highly contagious, and fatal disease that can affect a large number of freshwater and marine fish species.

In general, viral infections are often seen in koi, goldfish, and angelfish, but they can affect all the fish in the tank.


Viral infections are difficult to diagnose because their symptoms are similar to many other fish diseases. However, below are a few major symptoms that can relate to Viral Hemorrhagic septicemia-

  • Bleeding of the internal organs, muscles, and skin
  • Small red dots on the fish’s skin
  • Gills turning pale
  • Body turning dark colored
  • Fluid accumulation in the body
  • Bulging eyes

How to cure

There are no known treatments for viral infections. Generally, the fish suffering from viral hemorrhagic septicemia dies of the infection. If you are suspecting any fish of the viral infection, remove it immediately from the tank to prevent spreading the diseases to other fish. Keep the infected fish in the quarantine tank.

Clean the display tank thoroughly and increase the temperature of the water to a few degrees. You can treat the quarantine tank with antibiotics and anti-parasitic medications in case the infection is bacterial or parasitic and not a virus.

  1. Fish Lice

Fish lice are external parasites that attach to the fish’s skin and suck the blood. Due to fish lice, the fish gets anemic as they keep on sucking the blood. It may happen among both- freshwater and marine water fish.


  • Fish turn pale
  • Loss of fins and scales
  • Restlessness and lethargy
  • Increased mucus production
  • Erratic swimming
  • Poor body condition
  • Fish will rub against surfaces in an attempt to relieve the irritation mites causing
  • External wounds due to excessive rubbing

How to cure

The best way of getting rid of lice is by removing them manually from the fish’s skin. You have to use topical antibiotic ointment on the affected areas once the lice are removed. It will also help to heal the wounds and reduce the chances of bacterial infections at the site.

  1. Pop-eye

Pop eye occurs due to bacterial infection in which one or both the eyes of the fish pop out of the socket. Apart from the bacterial infection, the reason for the pop eye is the deficiency of Vitamin A, mass behind the eyes, and poor water quality.


Pop eye is easily diagnosable as the eyes abnormally protrude out of the eye sockets that confirms the fish is infected with the bacterial infection or has some underlying problem.

How to cure

First of all, take the medications from the pet store. Tetracyclines work most effectively against the pop eye. Later, you must check the water quality of the tank. It is advised to clean the entire tank and change the water to ensure that there are no more bacteria residing in it.

  1. Swim Bladder Disease 

The swim bladder is an important organ in the fish’s body that helps them to maintain buoyancy. The swim bladder contains oxygen and other gases so that the fish remain buoyant in the water. When the swim bladder is not functioning properly, it clearly means that the fish will face trouble maintaining the desired depth in the water.

It is observed that the swim bladder disease occurs when the water quality is not correct, and the fish is under huge stress. Many times the newly bought fish get this disease during transportation.


  • Fish will not be able to stay upright in the tank for long
  • Fish spending more time near the surfaces or deep in the tank

How to cure

The first thing you need to do when you notice such symptoms is taking your fish to the vet. The vet knows how to release the air from the swim bladder to correct the issue. After that, check the water quality and try to replace 10% of the water in the tank to further improve the quality.

If your fish is facing trouble staying upright, it will face trouble eating food too. It is when you need to learn how to hand-fed them. It will ensure that the fish is getting enough nutrition for survival. Many aquarists harness their fish upright to help them stay in the water properly.

  1. Tail and Fin Rot

The major reason behind getting tail and fin rot is unclean and improper water quality. Injury at any of the specified areas may also lead to the issue. When the water is dirty, the sites with cuts and abrasions easily get the infection. It will cause the areas to rot.


  • Fraying and torn fins
  • Ragged tail

How to cure

Since tail and fin rot is a common disease, you can easily find the medications for it at the shops. New methylene blue is an effective remedy that is recommended to cure tail and fin rot. It is best to put one drop per every 2 gallons of water. When using New Methylene Blue, you will need to do a 50% water change every other day for a week.


Catching infections and getting the diseases is uncertain; however, we can take certain preventive measures to avoid most of the health issues among the fish. The health of the fish majorly depends upon the quality of water you are providing. It directly correlates to the level of stress on the fish.

So, if you want to keep your fish healthy and happy and devoid of diseases and sickness, check out the below points-

Provide Quality Diet– Proper, well-scheduled, and healthy diet is the key to keep the fish healthy. When the fish is full, the body will get the required nutrition, and the immune system will work properly. Also, it will not get aggressive with other fish, and the mood will remain light.

Ensure high water quality- When the water in the tank is clean, and every device like power filters, etc., is working fine, the fish remain stress-free. You need to check all the water parameters from time to time to make sure that all are within their specified limit. When you maintain and keep your tank’s water parameters right, the chances of fish getting sick reduce.

Observe the tank routinely– Recreationally, you may often watch your fish swimming, but this is the time when you should start examining them critically. You need to check out where they are moving more (near the surface for gasping the air or in the deep layers), whether scratching their body to the sides of the tank or surfaces, how they are swimming, check out for the gills and fins, etc. All in all, you need to check if the fish is fine or under stress, and it can be detected easily with their behavior.

Water changes for reducing stress– It is advised to replace at least 10% of the water on a regular basis to reduce the chances of stress and diseases, if any. Moreover, if you find any fish being highly aggressive and bullying other fish, or you have just added a new one in your tank, make sure that other fish are fine and not stressed due to it.


Quarantine new fish– Often ignored, the quarantine step is highly crucial to keep most of the diseases at bay from your stock. You should always quarantine a newly bought fish as it could be a major source of most of the new parasites, bacterial, fungal, or viral infections. It increases the chances of the entire stock getting sick. Therefore, quarantining your new fish is essential to protect the stock from diseases.


It is not possible for any creature in the world to not getting sick. However, with certain measures, we can reduce the chances. As it is said, prevention is better than cure; make sure that you practice all the preventive habits to keep your fish at bay from sickness and stress.

However, if they still get the disease or show any signs of sickness, it is best to start the treatment right away. A lot of diseases get cured when detected in the early stages. So, do not wait for any further and talk to the pet store owners, fish vet, or any experienced aquarist to know the right ways of treatment.

Finally, keep the water parameters within the limit as it alone can prevent a lot of diseases and keep many issues away.

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