How to Cure the Most Common Fish Diseases

How to Cure the Most Common Fish Diseases

How to Cure the Most Common Fish Diseases

Is your fish showing signs that they are sick? Or does something just not seem right in your fish? Fish too can get sick. The most common reason that your fish could be sick is due to parasites. Your fish can have both internal and external parasites, causing them to have issues. They can also be afflicted with fungal, bacterial, or viral infections.

If you have noticed that your fish is just not right, it may be time to look for a fish vet. Yes, there are vets that also see fish. While it can be harder to find a fish vet, they do exist. If you are having trouble finding one, ask your local aquarium store, as they should know who will treat fish, and may even be able to give you advice on what to do to help your fish return to normal.

Most Common Freshwater Fish Diseases

These are some of the most common diseases that are seen in freshwater fish. You can see the video below from our YouTube channel if you want to have a visual presentation. We go into more detail in the blog post below.

1. Anchor Worms

Anchor worms are not actually a type of worm but a large parasitic crustacean from the Lernaea species that can live on your fish. Pond fish are most commonly infected with this species. They will attach to your fish’s skin and burry their head into your fish’s muscles.

Symptoms of Anchor Worms

If your fish has Anchor worms, common signs that you would see are red and inflamed scales. If you look close enough, you may even be able to see the body of these parasites sticking out of your fish. Their bodies will look like whitish green thread stuck to your fish. You may also notice that your fish is rubbing its body up against things in its tank.

Treatment of Anchor Worms

For larger fish, anchor worms can usually be easily removed by carefully pulling on the body of the parasite. After you have removed the parasites, you can apply topical antibiotic ointment to their scales. It would be best if you also cleaned the whole tank to get rid of any of the eggs, larvae, or parasites in the tank. For small fish in aquariums, using Hiraki USA Cyropro works without affecting your biological filtration.

2. Bacterial Infections

Your fish can get a bacterial infection. These commonly happen after your fish has injured a part of its body. A common bacteria for your fish to encounter is Aeromonas salmonicida1.

Causes of bacterial infections

Many times the cause of these bacterial infections is due to poor water quality and poor diet. This can cause stress in your fish leading to a lower immune system and allow any bacterial infection to set in.

Signs of a bacterial infection

If your fish has a bacterial infection, these are some of the most common signs that you will see:

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

Treatment of bacterial infections

If your fish has a bacterial infection, your vet will prescribe antibiotics based on the bacteria that is present. There are some medications that you can buy online or at a local aquarium store to help treat bacterial infections.

A popular brand for many fish vets to recommend is API’s Furan-2 or Triple Sulfa. These are medications that you put into their water. Many fish will need to be transferred to a quarantine tank or hospitalized tank while they are being treated. Before going back to their main tank, the issue with the tank would need to be corrected, and the tank cleaned.

3. Constipation

If your fish can not poop, they will quickly become constipated. This can cause your fish to become very ill.

Causes of Constipation

There are two main causes of constipation in fish

  1. Improper diet
  2. Parasite infections

Both of these can cause your fish to not be able to properly pass feces

Symptoms of Constipation

If your fish is constipated you may notice a bloated abdomen. Constipation can cause them to develop swim bladder disease so you will see signs of buoyancy issues.

Treatment of Constipation

One of the easiest ways to treat a constipated fish is to increase the amount of fiber in their diet and deworm your fish. Another way to treat constipation is to dose your tank with Epson salt. Adding 1 to 3 teaspoons of salt per every 5 gallons of water will help. Epson salt will act as a muscle relaxant and help them be able to poop easier.

4. CO2 Poisoning

CO2 levels over 30 ppm can be dangerous for your fish. This issue can arise with tanks that use CO2 injection in planted tanks.

Signs of CO2 Poisoning

If your fish’s tank has high levels of CO2 you will notice that your fish is breathing more rapidly and may be gasping for air. You also may notice that these fish are spending more time near the surface of the tank.

Causes of CO2 Poisoning

CO2 poisoning in fish can be caused by your CO2 reactor not working, or your plants not absorb CO2 due to the lights not working properly.

Treatment for CO2 Poisoning

The best way for you to treat CO2 poisoning in your fish is to use an air stone to agitate the surface. This causes the carbon dioxide to dissipate from the water. You can also adjust the rate of CO2 injection in your tank.

5. Flukes

Flukes are external parasites that your fish can get (Source- Cuttlebrook Koi Farm). Most flukes will affect fish gills. The two most common flukes that are seen in fish are Dactylogyrus and Gyrodactylus.ills

Causes of Flukes

These flukes can commonly enter your aquarium from another fish that is infected with flukes.

Symptoms of Flukes

Flukes will attach themselves to your fish’s gills and skin. This can damage these areas leading to a secondary bacterial infection.

Treatment of Flukes

The most effective treatment for flukes is Praziquantel. After you have treated these flukes, you can treat any wounds that they left on your fish with antibiotics.

6. Clamped Fins

Clamped Fins is just like it sounds, it occurs when your fish’s fins are clamped close to their body.

Causes of Clamped fins

Most fish will hold their fins very close to their body if the water condition is not correct. It could also be caused by external parasites on your fish.

Symptoms of Clamped Fin

If your fish has clamped fin, they may be lethargic and hold their fins very close to their body even while swimming.

Treatment of Clamped Fins

If your fish has clamped fins, it is best for you to check your water conditions. Also, look for any external parasites. If you see any parasites, you will need to manually remove the parasites.

7. Dropsy

Dropsy is a term used to describe your fish who is swelling due to kidney disease.

Symptoms of Dropsy

Usually, the signs of dropsy that are seen are a slightly swollen belly all the way to a very swollen abdomen so much that your fish’s scales will stick straight out, causing your fish to look like a pinecone.

Cause of Dropsy

There are many things that can cause your fish to have kidney issues. These are some common reasons:

  • Stress
  • Polycystic Kidney Disease

Treatment for Dropsy

If your fish has Dropsy, first start by putting it in a quarantine tank. This can help eliminate the stress that may be in the display tank if they improve while in the quarantine tank, there is a stressor in the main tank that needs to be addressed. This may be poor water quality, parasites, overcrowding, or something else. Try to figure out what is causing your fish to be stressed, and fix this issue.

Dropsy is a serious disease that lead to the death of your prized pet. It is best for you to see a vet. Some vets will see fish; however, it may be hard to find one near you. Reaching out to your local aquarium store, you may be able to get suggestions on who to see to help treat your fish.

I know both of these are not always available to some, so I’m going to defer to Lori’s Hartland’s experience on how she cured her Goldfish using a triple treatment of KanaplexMetroplex & Epsom Salt. Here is the full video below that walks you through her experience. It’s also very important to see her experience with her vet call at 19:06. She had to take a phone consultation, which is going to be what most will get when consulting a vet for smaller fish.

8. Freshwater Ich

Ichthyobodo is a protozoan parasitic infection seen in fish who are stressed.

Symptoms of Ich

Ich attacks your fish’s gills and skin. These parasites will cause your fish to have a grey color to their skin. You will notice that your fish will be lethargic, weak, and not want to eat. You may notice that they spend more time near the top of the tank, gulping air or rubbing their side on their tank.

Cause of Ich

Stress is the main cause of Ich. This can be due to poor water conditions, overcrowding, or illness. When your fish is stressed, it leads to a lower immune system and allows this protozoal parasite to take over.

Treatment for Ich

If your fish has Ich, it would be best to see a vet or an aquatic specialist. They will help guide you on how exactly to treat your fish. Common things used are medicated fish tanks with aquarium salt, potassium permanganate or copper sulfate. Seeking the help of an experienced fish hobbyist or a vet who will treat fish will help make sure that you are not going to harm your fish.

In Mark’s experience, his drug of choice has been Ich-X for freshwater fish. He often says the freshwater side of the hobby has it much easier with a wonder solution like Ich-X. Watch for secondary infections, as these will usually kill the fish.


9. Freshwater Velvet (Gold Dust Disease)

Velvet in fish can be very deadly for your whole tank. This disease can quickly wipe out everything in your tank.

Symptoms of Velvet

Fish who have velvet will be scratching their body against any hard surface in the tank. This is to try to remove the parasites from their skin. They may also display some of the following signs:

  • Lethargic
  • Rapid Breathing
  • Not eating and weight loss
  • Holds fins next to body
  • Labored or rapid breathing
  • Yellow to rust color dust on the fish’s body
  • Pealing of the skin in severe cases

Causes of Velvet

Velvet is caused by the parasite Oödinium pillularis or Oödinium limneticum. These parasites may be present in many aquariums but only cause issues if your fish is stressed, sick, being transported, has a sudden, temperature change or has poor water quality.

Treatment of Velvet

If your fish has velvet, you do a few things to help treat your fish.

  • Increase the temperature of the water by just a few degrees
  • Dim the aquarium lights for a few days
  • The treatment of choice for velvet is copper sulfate for 10 days. A good brand to look for is Copper Power or SeaChem CupramineDO NOT use copper sulfate in a displayer tank. A removal of the sick fish to a quarantine tank is a must!

Velvet can many times be prevented by quarantining any new fish that you are planning on adding to your tank, providing your fish with a proper diet, and maintaining good water quality. If you came here to get info on the saltwater version, check out Mark’s article on Marine Velvet.

10. Fungus

Fish can get fungal infections. Two common fungal infections seen in freshwater fish are:

  • Saprolegnia2and
  • Ichthyophonus hoferi 

Symptoms of Fungal Infections

A Fungal infection can cause damage to internal organs. Common signs seen in fish with fungal infections are grey cotton-like growths seen on the skin, gills, fins, and around the eyes.

Causes of Fungal Infections

Fungal infections are caused by unclean water conditions and dead and decaying organic material in your tank.

Treatment of Fungal Infections

The first thing you need to do to treat fungal infections in your fish is to clean their tank fully. Then using potassium permanganate that you put into your fish’s water. You will want to make sure that you have removed any external pathogens from your fish’s skin. A popular brand of fungal treatment that you can use after fully cleaning your fish tank is API Fungus Cure.

You can also increase the water temperature to 82 degrees as most of these fungal infections thrive in colder temperatures.

11. Gill Mites

Gill mites3 are common parasites that are seen in fish. These parasites attach to your fish’s skin and feed on their blood.

Symptoms of Gill Mites

If your fish has gill mites, you will notice that their gills do not fully close. These mites prevent the gills from being able to function properly. This will cause your fish to spend most of its time at the surface of the tank gasping for air.

Causes of Gill Mites

These mites enter your tank from new fish who already have these mites. This is one reason why it is advised to always quarantine any new fish before adding them to your aquarium.

Treatment of Gill Mites

To treat gill mites you will need to use sterazin and octozin. You will need to repeat this treatment about 5 days later to kill any other mites that may have hatched.

12. Hole In The Head

Hole in the head disease is seen when there are small indentions in your fish’s head or along their lateral line.

Symptoms of Hole in the Head

If your fish has hole in the head, you will start to see small indentions into their skin. These will start as slight depressions and can advance to more severe holes. Some fish with this disease will stop eating.

Causes of Hole in the Head

Hole in the head is caused by a a protozoan called Hexamita. It is a common disease in Discus fish. Another factor is deficiency of minerals in water due to the use of RODI water or excessive usage of activated carbon.

Treatment of Hole in the Head

The best way to treat this is in a quarantine tank and treating with Metronidazole or API General Cure.

13. Hemorrhagic septicemia

Hemorrhagic septicemia is a deadly viral infection seen in fish caused by Piscine novirhabdovirus.

Symptoms of Hemorrhagic septicemia

Fish with this disease will have bleeding in their internal organs, muscles, and skin. This can be seen as small red dots on your fish’s skin. You can also see some of the following symptoms:

  • Pale gills,
  • Dark-colored body,
  • Fluid accumulation in the body,
  • Bulging eyes

Treatment of Hemorrhagic septicemia

There is no really good treatment for this disease. Many fish who get this disease will die of this infection. You can try to clean the tank and increase the temperature of the water that the fish are in a few degrees. Medications like API Triple Sulfa, Kanamycin, and Thomas Labs Fish Mox can also be effective.

14. Lice

Your fish can get external parasites such as lice. These small parasites can attach to your fish’s skin and suck blood causing your fish to become anemic.

Symptoms of Lice

If your fish has lice you may see a small clear disc laying next to your fish’s body. Most fish who have lice infestation are pale due to the lice sucking blood. They may also be rubbing against surfaces in the tank trying to remove these fish. This can cause them to have external wounds on their skin.

Treatment of Lice

The best way to treat lice in fish is to physically remove all the lice from your fish’s skin. If there are any external wounds, you can treat these areas with topical antibiotic ointment. Over the counter medication of choice would be diflubenzuron under the brand name Dimilin-X.

15. Pop-Eye

Pop-eye is due to a bacteria infection that causes one or both eyes of your fish to pop out of the socket.

Symptoms of Pop-Eye

This condition is usually very easy to diagnoses as your fish will have their eyes abnormally protruding from their eye sockets.

Causes of Pop-Eye

Pop-Eye is usually due to a bacterial infection. It can also be due to vitamin A deficiency, mass behind the eye, or poor water quality.

Treatment of Pop-Eye

Pop-eye is commonly treated with tetracyclines. You can get this medication from your local veterinarian who sees fish. Also, you will need to check the water quality in your tank and fully clean their tank. If you cannot get a medication from a vet, you can try over the counter sulfa drugs like SeaChem SulfaPlex.

16. Swim Bladder Infection

The swim bladder is an organ in your fish that contains oxygen and other gases so that your fish is buoyant. If their swim bladder does not function properly, your fish will have trouble maintaining their desired depth.

Symptoms of Swim Bladder

One of the main signs that you will see in fish with swim bladder disease is that your fish has a hard time staying upright in their tank. They may also spend a lot of time near the surface or deep in the tank.

Causes of Swim Bladder

One of the most common reasons your fish could have swim bladder disease is poor water quality and stress. Newly purchased fish may get this disease if transported incorrectly. If you notice that your fish cannot maintain proper buoyancy, the first thing your need to check is the water quality in your tank.

Treatment of Swim Bladder

If your fish does have swim bladder disease, it would be best for your fish to see an aquatic vet. An aquatic vet is able to perform a procedure to release the air from the swim bladder to correct the issue. They will also recommend that you test your tank’s water quality and perform water changes of about 10%.

Fish who have trouble staying upright or spend a lot of time near the tank’s bottom may need to learn how to be hand-fed. This will help ensure that your fish is getting the proper nutrition that it needs to survive. Hobbyists have also tried making harnesses to keep their fish upright, like the video below. Check out Mark Kennedy’s video for the full walk-through. The harness solution is most effective with fancy goldfish.







17. Tail and Fin Rot

Causes of Tail and Fin Rot

Fin rot is usually caused by unclean and improper water quality and an injury to one of these areas. This cut or abrasion can then easily get infected due to unclean water. This can cause these areas to rot.

Signs of Tail and Fin Rot

In fish with these issues, you will see fraying and torn fins and a ragged tail.

Treatment of Tail and Fin Rot

There are medications that you can get to treat these infections, such as SeaChem KanaPlex and API Fin and Body Cure. You can also use New Methylene Blue to treat your fish tank. It is best to put 1 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.

18. Mouth Rot

Mouth rot is caused by a fungus called Flavobacterium bacteria.

Signs of Mouth Rot

If your fish has mouth rot, you will see a white cotton-like appearance around their mouth. If this is left untreated, the skin around your fish’s mouth will start to rot.

Causes of Mouth Rot

This is usually caused by unkept water conditions and stress. This lowers your fish’s immune system causing fungus to take over.

Treatment of Mouth Rot

There are many antifungals that you can buy at your local aquarium store to help treat this issue such as SeaChem KanaPlex, Thomas Labs Fish Min, and API Fin and Body Cure. This will be a medication that you add to their tank. You should also try to figure out what is causing your fish to be stressed and check your water quality.

How to prevent Many health issues

There are many things that you can do at home to help prevent many of these issues in your fish. Your fish’s health depends on water quality, diet, and levels of stress. To help keep your fish healthy, these are a few things that you can do to help your fish stay happy and healthy.

  1. Provide a Quality Diet: Your fish should be fed a proper diet every day. Diet will help keep your fish full and not fight with other fish in the tank over food.
  2. High Water Quality: Make sure that the water that your fish is living in is clean using high quality power filters or canister filters, and at the correct temperature. Also, check to make sure that the levels in your fish tank are correct. By maintaining and keeping your tank at proper water parameters will help keep your fish healthy.
  3. Observation of tank: It is best to watch your fish in your tank. This will let you know if your fish is sick or stressed. Early detection of a disease is easier to treat than one that is not detected until later in the disease stage.
  4. Reducing Stress: Make sure that you reduce the stress in your tank. If you have a fish that is bullying other fish or you have just added a new fish to the tank, make sure that your fish are not stressed.
  5. Quarantine New Fish: Any new fish should be quarantined for a few weeks to make sure that you are not putting a sick fish into your tank. This gives you a few weeks to make sure that your fish is eating and not showing any signs of illness before putting them in your main tank and cause your other fish to also stay healthy. Mark has a great article on quarantining. It is marine fish based, but the same principle applies for freshwater fish.

Creating A Medicine Cabinet

Anyone who has multiple freshwater fish and tanks deals with sick fish. Whether you quarantine or not, it’s a based practice to have medication on hold for emergencies. Here would be common readily available medications that are helpful to have on hand. To summarize all the medications discussed, I have added a table for quick reference.

Type Medications
Parasitic (External) Ich-X, Prazipro
Parasitic (Internal) General Cure, Seachem Metroplex, SeaChem Focus (To Bind)
Fungal API Fungal Cure
Bacterial API-Furan 2, Seachem Sulfaplex


What are the most common issues?

The most common fish diseases are Ich, internal parasites. Dropsy is common in fancy goldfish. Hole in the Head disease is common with Discus fish, and Fin Rot is problematic with Betta fish.

How Do You Identify Different types of Infections?

The most obvious way to identify a fish disease are external signs. You may notice odd behavior in the fish (such as flashing, scratching), cloudy eyes, or a whitish coatings on its body. Another obvious sign is a skinny fish that is eating, a common sign for a fish inflicted with an internal parasite. Identifying a fish disease is critical to treating them in time.

Can You Get Heath Issues From Them?

No. You cannot get a disease from an aquarium fish unless you eat them. Since we do not eat our aquarium fish, getting ill from keeping fish tanks is very rare. As long as you care for your fish and clean your hands after putting your hands in your tank, you should not have any concerns with getting sick due to keeping a fish tank.

How Do you Deal With Health Problems?

Dealing with fish disease involves treatment. You must first know what type of disease you are dealing with – parasitic, bacterial, fungal, or internal. Each type has different ways of treating it. There are many store-brand medications these days. For large fish, Vets can prescribe medication. This is more common for large fish like Koi and Cichlid while small fish a Vet will consult with you for guidance.

How Do I Know If they have Internal Parasites?

The most obvious way to know if your fish has an internal parasite is weight loss. If your fish is eating and says skinny it likely has an internal parasite. The other is the shape and color of the fish poop. If you see stringy white poop, your fish has an internal parasite. Most internal parasites can be treated with Metroplex

Final Thoughts

If your fish seems like they are sick or displaying signs of parasites, it would be best to start treatment right away. Many of these diseases can be easily and quickly treated if caught early. The best way to treat any issue in your fish is to prevent them from happening. By keeping the water parameters in your tank in ideal ranges, and keeping your tank clean, you can prevent many of these fish illnesses.



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