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How To Spot And Treat 23 Of The Most Common Fish Diseases

Here are 23 of the most common aquarium fish diseases. We tell you how to spot the main symptoms, what the causes are, and how you can best prevent and treat them.

By Andy Birks
Last updated on

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fish diseases

Whilst fish diseases are never nice to encounter, I’ve unfortunately seen many of the ones below in tanks over the years.

Often resources online can lack clear photos to help you identify symptoms and what to look out for. I wanted to fix that.

If you keep a well-filtered, well-maintained tank, and make sure you quarantine new fish you’ll hopefully avoid seeing many of these. But it never hurts to be prepared.

So keep reading to find out how to prevent the main fish diseases in your tank, how to spot the symptoms, and how to treat them.

Fish Diseases: Main Types And Prevention

Summary of fish disease types with examples

There are numerous fish diseases that can affect aquarium fish. Let’s start by breaking them down into the main categories:

You’ll see I’ve included a final section on ‘Other Issues’. These are symptoms rather than specific fish diseases in themselves. But they are still important to be aware of and have the potential to occur in your fish.

Preventing Fish Diseases In Your Tank

There are several ways you can prevent many of these fish diseases from happening in your aquarium.

  • Provide A Quality Diet: Knowing each species in your tank and providing them with the right diet will go a long way to preventing them from getting ill. Whether herbivore, carnivore, or omnivore, make sure you give each type of fish the right, high-quality food. 
  • Monitor And Maintain Good Water Conditions: Poorly filtered and maintained water is one of the main causes of many of the fish diseases below. I can’t stress enough the importance of a balanced filter in your tank. And by this I mean one that has an established colony of nitrifying bacteria. This will work to remove toxic ammonia and nitrites, keeping the water clean and safe for your fish.
  • Observe Your Fish Regularly: One of the best ways of catching fish diseases early is by knowing your fish. Keep an eye on them on a daily basis. A good time is when you feed them. Look out for anything out of the ordinary. Whether that’s physical signs or changes in their behavior.
  • Reduce Stress On Your Fish: Stress is another big factor that can lead to fish diseases in your tank. Stressed fish often have lower immune system function. Don’t overstock your aquarium, and if any fish are fighting or being bullied you may have to think about re-homing some. Always keep an eye on your water conditions too. Things like sudden temperature changes, for example, can cause significant stress.
  • Quarantine New Fish: I’d always recommend quarantining any new fish you add to your tank. This is one of the best ways you can check they are not carrying any infections before introducing them to your aquarium.

If you do have to administer medications in your tank make sure you remove any activated carbon from your filter. Activated carbon removes chemicals from your water – that will include any medicines you add!

Parasitic Fish Diseases

1. Ich (White Spot)

Symptoms To Look For

  • Very circular, small, raised, white spots similar to grains of salt.
  • Spots appear evenly across the gills and body, rather than in clusters.
  • Fish may scratch against objects due to irritation.
  • May gasp for air at the surface of the tank.
  • May clamp fins.


Ich (or White Spot) is one of the most common parasitic fish diseases in aquariums and can affect most fish. Whilst it is very treatable it is highly contagious. 

Caused by the protozoa Ichthyophthirius multifiliis, it is mainly triggered by stress lowering your fish’s immune system. Poor water conditions, sudden temperature changes, or overcrowding can all contribute to this.


Try to make sure that your fish do have Ich before you treat them. If the spots aren’t evenly distributed and raised like salt grains then it could be a different condition like epistylis. 

Some people advise that you increase the temperature in your tank to treat Ich. All this will do is speed up the parasite’s lifecycle so it falls off your fish quicker. It will still be in your tank. 

To treat Ich you need to use a white spot medicine for around 14 days to ensure all of the parasites and carrier cysts have been removed. Ich-X or API White Spot Cure contains malachite green which has been shown to effectively treat the parasite [1], [2].

2. Hole In The Head

Symptoms To Look For

  • Pits or ulcers around the head.
  • Pits along the lateral line which runs around the jaw and eyes.
  • Fish can become paler and lose color.
  • Can become lethargic.


Hole In The Head can affect freshwater and saltwater fish. Cichlids are particularly prone to this fish disease, especially oscars and discus. It is often caused by the parasite Hexamita sp. and is also known as hexamitosis [3].

Causes can include stress brought on by bad water conditions and feeding your fish poor-quality food [4]. It has also been shown that using granulated activated carbon (GAC) in your filter may contribute to the disease [5].


Hole In The Head is treatable if caught early, but becomes much more difficult if left too late. As soon as you see any signs of ulcers you should isolate your fish in a quarantine tank.

Use a medicine such as API General Cure which contains metronidazole. This active ingredient is one of the most effective treatments for Hole In The Head.

3. Neon Tetra Disease

Symptoms To Look For

  • Growths or cysts especially near the tail fin.
  • Loss of color or brown tint to areas of the body.
  • Fish may become skinny.
  • Swimming abnormally.


Unfortunately, this is one of the more serious fish diseases and is usually fatal. It can affect several different types of tetras, barbs [6], and some danios [7].

Neon Tetra disease is caused by the parasite Pleistophora hyphessobryconis. The parasite infects the digestive tract and leads to secondary bacterial infections. This can cause great damage to the internal organs.


Sadly, there is no effective treatment for this fish disease. If you find an infected fish in your tank you should remove it immediately and euthanize it to prevent the spread. Carry out a 50% water change and thoroughly clean your tank too.

4. Anchor Worm

Symptoms To Look For

  • Worm-like parasites visibly attached to the body or gills.
  • Inflamed scales, particularly where the worms are attached.
  • Fish may scratch against objects.
  • Can become slow and lethargic.
  • May show signs of labored breathing.


Rather than worms, these parasites are actually crustaceans called Lernaea cyprinacea [8]. The young larvae start life by feeding on blood and cells in your fish’s gills [9].

Anchor Worms can infect most species, but they are more often found in koi or goldfish in larger aquariums or ponds. They can be introduced when you unintentionally bring home new fish that are already infected.


Although Lernaea are not usually fatal to fish, they can lead to bacterial or fungal infections that may be more serious.

As the parasites are large, the best way to treat them is to carefully remove them with forceps. Just make sure you remove the whole parasite as sometimes the head can break off and be left in your fish.

There are also various antiparasitic products that you can use to treat your fish. The most effective are those that contain potassium permanganate [10]. You can often use this as a dip or to treat the tank. But check the individual product as it is a strong chemical and some fish can be sensitive to it.

5. Velvet (Gold Dust Disease)

Symptoms To Look For

  • Small gold or rust-colored dust-like specks on the head and body.
  • Fish may rub against objects due to irritation.
  • Lack of appetite and fish may become skinny.
  • Clamped fins.
  • Labored breathing and lethargic.


Velvet, or Gold Dust, is one of the more common and serious fish diseases that can affect freshwater and saltwater fish. It is caused by Oodinium spp. parasites that may be present in your aquarium but only affect your fish if they get stressed. 

Causes of stress can include sudden temperature changes, poor water quality, or adding new infected fish without quarantining.


Oodinium parasites use photosynthesis to provide some of their food [11], and so one way to treat them is by keeping the tank in darkness. You can also speed up the parasite’s lifecycle by raising the temperature in your tank to around 82° F. But bear in mind some fish aren’t as tolerant to higher temperatures and this will also lower oxygen levels.

The best treatment is to use copper-based medication [12]. Products such as SeaChem Cupramine are a good example. You will need to treat your tank for around 10 days, but do follow the product’s instructions.

6. Argulus (Fish Lice)

Symptoms To Look For

  • Sometimes visible as tiny dark oval lice around the head or pectoral fins.
  • Fish may scratch against objects to try and remove the parasite.
  • Inflamed areas where the lice have attached.
  • Erratic swimming and darting.


Argulus spp. are one of the biggest parasites affecting marine and freshwater fish. They are generally treatable although heavy infestations can be fatal [13].

You may risk bringing them into your aquarium via infected fish if they are not properly quarantined. Once born the larvae become immediately parasitic. They will attach to your fish, going through several molting stages as they become adults [14].


You should start by carefully removing the lice from your fish with forceps. You can then treat with a diflubenzuron-based medication [15], such as Dimilin-X.

7. Roundworms

Symptoms To Look For

  • Fish may become sluggish and swim slower.
  • Lack of appetite.
  • Bloating or sinking in of the abdomen.
  • Lumps on the body.
  • White or pinkish worms hanging from the anus in severe cases [16].


Roundworms are nematodes [17] and can affect any fish in your aquarium, but are often a problem for guppies. They affect the digestive tract and internal organs and can cause severe illness.

This fish disease is often caused by adding infected material to your tank. It could be new infected fish, live food, or other organic material. A dirty tank with uneaten food can also lead to roundworms taking hold.


Praziquantel is an effective medication against roundworms, especially when soaked into food. You can find this in products such as PraziPro. Fenbendazole and levamisole have also been shown to be effective roundworm treatments [18].

Thomas Labs Fish Bendazole and Fritz Expel-P are good examples of products that work using these active ingredients.

8. Skin Or Gill Flukes

Symptoms To Look For

  • Difficulty breathing or gills moving abnormally.
  • Inflamed or red areas, particularly around the gills.
  • Fish may scratch against objects in your tank.
  • Excess mucus on the gills or body.
  • Clamped fins or stomach sinks in severe cases.


Skin Flukes (Gyrodactylus spp.) and Gill Flukes (Dactylogyrus spp.) are tiny parasites common in saltwater and freshwater fish. Guppies can be particularly prone to them [19]. Unfortunately, flukes are nasty parasites and can be fatal in many cases [20].

A lower immune response in your fish caused by stress is the main thing that leads to fluke outbreaks. As with other fish diseases, this can be caused by poor-quality foods, bad water conditions, overcrowding, or adding new fish to your tank without quarantining.


Similar to roundworms, you can treat flukes with medicines containing praziquantel. This is a strong medication though and not suitable for use with some aquatic life. So always follow the dosage and other instructions. If you catch an outbreak early enough you can use products such as API General Cure.

9. Gill Mites

Symptoms To Look For

  • Reddening around the gills.
  • Fish may gasp for air at the surface.
  • Gill covers may stay slightly open.
  • Fish may scratch against objects.


Gill mites are ectoparasites meaning they live on the external surface of their host [21], [22]. The mites are invisible to the naked eye and feed on the blood and flesh of your fish’s gills.

They are usually introduced by bringing infected fish into your aquarium. If your fish is stressed it can also make them more susceptible [23].


Again praziquantel-based medicines, such as PraziPro, are strong but effective against gill mites. Always follow the directions but you will need to treat your fish for at least 5 days.

You fish can also often pick up secondary bacterial infections if infected with gill mites. So it’s best to treat your tank with a general cure or antibiotics.

Bacterial Fish Diseases

10. Fish Tuberculosis

Symptoms To Look For

  • Loss of coloration.
  • Stomach may sink in.
  • May develop ulcers or dark areas on the body.
  • Fish may become sluggish and have difficulty swimming.
  • May see curvature of the spine in severe cases.


Fish tuberculosis can be caused by several species of mycobacterium, but the most common is Mycobacterium marinum [24].

This bacterium thrives in water that is low in oxygen and low pH. So if you don’t properly clean your tank, monitor your water parameters, or keep up with water changes you risk creating these conditions.


Unfortunately, there is no treatment currently available for this fish disease. You can try strong antibiotics in the early stages. But generally, your only course of action would be to euthanize your infected fish and sterilize your tank.

Note: Fish tuberculosis can be passed on to humans in rare cases, particularly if you have any open wounds [25]. Always wear rubber gloves, wash your hands, and sterilize equipment if you are dealing with this fish disease.

11. Fin Or Tail Rot

Symptoms To Look For

  • Decay at the edges of the fins.
  • Edges may appear white or faded.
  • Fish may spend more time at the bottom of the tank.
  • May have difficulty swimming.


Fin rot can be caused by several bacterial species with one of the most common being Aeromonas spp. [26]. 

Housing your fish with incompatible tankmates can often lead to this fish disease. Any fin nipping by aggressive fish can lead to bacterial infection and fin rot. Stress can also be a key factor. This can be caused by overcrowding or poor water conditions.


Fin rot is a curable fish disease but there are certain steps you should take. Firstly, improve your water quality by performing a 25 to 50% water change and siphoning your gravel. You’ll also have to find new homes for any aggressive fin nippers in your tank.

Some of the more effective antibacterial treatments for this fish disease contain tetracycline (such as API Fin And Body Cure) or kanamycin (such as Seachem Kanaplex). These active ingredients offer good levels of treatment against Aeromonas spp. [27].

12. Mouth Fungus (Columnaris)

Symptoms To Look For

  • Ulcers on the body.
  • Cloudy fungus-like patches on the body, gills, or mouth.
  • Eventual rotting of gills or mouth.
  • Fish may have difficulty breathing.
  • Loss of appetite.


Whilst often called mouth fungus, this fish disease also affects the body and gills, and is caused by the bacterium Flavobacterium columnare [28]. It is highly contagious and can affect all fish, but it is often found in livebearers.

Columnaris can be caused by a poor diet, bad water conditions, overcrowding, and other conditions that cause stress for your fish.


If this fish disease becomes advanced it can be fatal. But if you catch it early there are several steps you can take. Firstly, check your water parameters and carry out water changes as needed. F. columnare thrives in high nitrite levels so keeping these down at zero is key [29]. 

If your fish can tolerate aquarium salt you can add this to the water. Salt can help reduce transmission of F. columnare [30]. You can also treat with antibacterials such as API Fin And Body Cure or Seachem Kanaplex.

13. Cloudy Eye

Symptoms To Look For

  • Gray or white cloudiness across the eyes.
  • Fish may suffer loss of vision.
  • May swim erratically.


Cloudy eye is one of the most common fish diseases in freshwater fish [31]. There are many possible causes including poor diet, parasitic or bacterial infection, or simply old age.

Water quality is one of the most common causes, particularly where your pH is too low.


Start by testing your water and if the pH is lower than 6 you can raise it using crushed coral or marine salt. You should also perform a couple of water changes and continue to monitor the parameters. 

You can use treatments like API Melafix or stronger medication such as Seachem Kanaplex to treat the tank.

14. Enteric Redmouth

Symptoms To Look For

  • Mouth and throat become red and inflamed.
  • Gills and fins can also redden.
  • Stomach may show signs of redness.
  • May have lack of appetite.
  • Fish may become lethargic and have difficulty swimming.


Enteric redmouth is a fish disease of the mouth, digestive tract, and liver caused by the bacterium Yersinia ruckeri [32], [33]. It’s a cold water disease so is rare in tropical aquariums, but if you own a pond with koi or goldfish there’s a chance you could encounter it.


Whilst this is a rare disease, unfortunately, it’s often untreatable when it gets to more advanced stages. If you catch it early enough you can use API Melafix or try stronger antibiotics.

Viral Fish Diseases

15. Lymphocystis

Symptoms To Look For

  • Cauliflower shaped cotton-like patches on the head and body.
  • White areas around the eyes.
  • Swimming may become erratic.
  • Fish may have difficulty breathing if several growths are on the gills.
  • May become slow and lethargic.


Lymphocystis is a common viral infection in freshwater and saltwater fish. It is caused by the iridovirus lymphocystis disease virus (LCDV) [34]. It can particularly affect bony fish including cichlids, killifish, and gouramis [35]. 

This fish disease affects the skin and fins and is not generally serious enough to be fatal, although it can cause deformities.

Lymphocysits is caused by bringing new fish into your tank that are already carrying the infection. So lack of proper quarantining is a key factor. But it can also be introduced by infected live foods.


There are currently no medicines available to treat this fish disease. But in most cases, your fish’s immune system will destroy the virus within a month. You can aid this by keeping your water conditions monitored and in excellent condition, and making sure your tank isn’t overcrowded.

You can also help prevent a secondary bacterial or fungal infection by treating your tank with the appropriate general cure.

16. Hemorrhagic Septicemia

Symptoms To Look For

  • Bruising or bleeding visible under the surface of the skin.
  • Fish’s eyes may bulge.
  • Bloated stomach.
  • Ulcers or open sores on the body.
  • Erratic swimming and gasping at the surface.


Hemorrhagic septicemia can be caused by bacterial or viral infection. In this case, I’m looking at the viral hemorrhagic septicemia virus (VHSV), also known as Egtved virus [36]. It is a serious disease and usually fatal, particularly as the internal bleeding it causes can lead to dropsy. 

As with Lymphocysits, this fish disease can make its way into your tank via new fish that are already infected. If you have poor water conditions, particularly an ammonia spike, this can help the disease thrive too.


Unfortunately, there are no known treatments for this fish disease. Prevention is certainly better than cure in this case. If you have an outbreak in your tank you are likely to need to euthanize the infected fish and thoroughly disinfect your tank.

Fungal Fish Diseases

17. Body Fungus

Symptoms To Look For

  • Cotton-like strands growing from the head, body, or fins.
  • Strands usually white but may take on brown or greenish color.
  • Fish may also suffer slight discoloration.
  • May become lethargic.
  • Slow or erratic swimming.


Often caused by Saprolegnia spp. [37], body fungus can take hold if your fish has open wounds or damage to its skin. 

If your fish comes under stress from temperature changes or poor water conditions it can also increase the likelihood of body fungus.


If you maintain good water conditions and a clean tank, you are unlikely to get an outbreak of this fungus. As long as your fish are in good health the mucus layer on their skins will stop the fungal spores from taking hold.

But if you do get an outbreak of this disease you should treat it without delay as it can spread. There are many commercial medications that work well against body fungus. These include Ich-X, and API Primafix Antifungal Freshwater and Saltwater Fish Remedy. But do check the product as some are not suitable for all fish.

18. Cotton Fin Fungus

Symptoms To Look For

  • Cotton-wool like growths on the fins.
  • Growths appear very white in color.
  • Dead tissue underneath the growths may show signs of rot.
  • Fish may swim slowly or erratically.
  • May scratch against objects in the tank.


Often known as Cotton Wool Disease, this fish disease is also caused by Saprolegnia spp. It can appear similar to the bacterial disease Columnaris, and can be difficult to tell apart if found around the mouth of your fish. However, Cotton Fin results in very white fungal growths whereas Columnaris can be gray and duller. 

Cotton Fin is often caused by an excess of food or rotting material in your tank, leading to poor water conditions. Older fish or those with weaker immune systems can be more susceptible.


Antifungal mediations such as Ich-X or API Primafix Antifungal Freshwater and Saltwater Fish Remedy are very effective against this fish disease. Dip treatments with a copper-based medication for around 1 minute have been shown to be effective too [38].

You should look to improve your tank conditions through regular water changes and checking your filter is in good working order. In many cases, Cotton Fin will resolve itself when the water quality improves.

Other Issues

19. Swim Bladder Issues

Symptoms To Look For

  • Unable to stay upright in the water.
  • May have difficulty swimming to the surface or the bottom.
  • May swim erratically.
  • Fish may have swollen stomach.
  • May become slow and lethargic.


This is a symptom rather than a fish disease. Swim bladder issues can happen in any of your fish, but they are common in goldfish, balloon mollies, and bettas.

Your fish’s swim bladder is an organ with a pneumatic duct that it uses to control its buoyancy in the water. Issues can be caused by things like digestive problems, or bacterial, or fungal infections [39]. They can also be caused if your fish sucks in too much air when surface feeding.

The most common cause is poor water quality causing stress in your fish. Often swim bladder issues can actually be lethargy due to difficulty breathing in poorly oxygenated water.


Firstly, check your tank’s water quality and perform a 10 to 25% water change. If you don’t see any improvement in the first week, treat your tank with aquarium salt or a general cure.

I’d always recommend consulting a vet if you don’t see improvement at this stage. Swim bladder issues are very difficult to diagnose and until you know the cause it’s almost impossible to treat.

20. Dropsy

Symptoms To Look For

  • Very swollen abdomen.
  • Swelling may cause scales to stick out.
  • Loss of appetite.
  • Rapid or labored breathing.
  • Fish may be slow and lethargic.


Dropsy is actually a symptom of an infection rather than a specific fish disease. It is a bacterial infection of the kidneys causing fluid buildup and extreme bloating. In most cases, dropsy is caused by the bacteria Aeromonas spp. [40].

Overcrowding, poor water quality, or rapid changes in water temperature can allow these bacteria to thrive in your fish [41].


This is a very serious fish disease that unfortunately is usually fatal. Even if you catch it early the mortality rate is often high.

As soon as you notice any symptoms move your fish to a quarantine tank. You can try to treat with strong medications such as Seachem Kanaplex. But if you suspect dropsy I would recommend contacting your vet straight away.

21. Pop-Eye

Symptoms To Look For

  • One or both eyes swollen and sticking out.
  • Eye may become cloudy, bloodstained, or ruptured.
  • Fish may lose vision.
  • Fish may appear lethargic.


Pop-eye is another symptom rather than a specific fish disease and can have a number of causes. Your fish’s eye will protrude out due to a build-up of fluid or sometimes a mass behind the eye. Often a bacterial or occasionally a parasitic infection can lead to pop-eye [42].

Physical damage, fighting, or poor water quality can all contribute to the conditions for these infections to take hold.


If you think your fish has pop-eye it’s best to first move it to a quarantine tank. 

Check there are no sharp objects in your tank that could have caused damage. Also, test your water and perform a water change if the parameters are too high.

You can treat pop-eye with antibacterials similar to those used to treat fin rot. Tetracycline-based products are a good option.

22. Ulcers

Symptoms To Look For

  • Sores on the head or body.
  • Wounds can be open with the skin peeled away.
  • Fish may rub against objects due to discomfort.
  • May swim erratically.


Skin ulcers can occur in any of your fish and are one of the most well-recognized signs of a stressed aquatic environment [43]. 

They can be caused by things like physical injury, bacterial or parasitic infection, or poor water conditions.


If your fish develop ulcers, quarantine those affected and perform a high water change of around 40 to 50%.

You can also use treatments such as API Melafix or a stronger medication like Seachem Kanaplex to treat your fish and tank.

23. Ammonia Or Nitrite Poisoning

Symptoms To Look For

  • Gills may look swollen or red in color.
  • Fish may gasp for air at the tank’s surface.
  • May swim erratically.
  • Fish may become slow and lethargic.


Although not a fish disease, ammonia or nitrite poisoning can be common in home aquariums. It often happens when you first set up a new tank. If you’ve introduced your new fish before cycling the aquarium properly you could subject them to poisoning [44].


Check the quality of your water and make sure ammonia and nitrites are down to the correct levels, which is usually 0ppm. A new tank will need between 3 to 6 weeks to cycle and allow nitrifying bacteria to establish in your filter properly.

If you already have fish in your tank, check your ammonia, nitrite, and nitrate are at the right levels. If not, perform a larger water change (around 40%) and keep up with regular changes until you bring the levels down.

The Bottom Line

Whilst some of the fish diseases we’ve gone through here are serious, the majority are treatable especially if caught early.

You can prevent many of these from happening in the first place by keeping a clean and well-maintained tank. Monitor your water conditions on a regular basis, don’t overstock, and be careful to quarantine any new fish.

Focus on these preventative steps and you’ll be able to keep a happy and healthy tank. So hopefully you won’t see many of the fish diseases here.

Important note: Our recommendations are based on extensive experience and research supported by reliable references. However, this does not represent veterinary advice. Always consult your veterinarian if you have concerns about your fish’s health.

*Images are for illustrative purposes.

Photo of author
Andy Birks
Andy is the owner of Simply Aquarium and manages the team of experienced writers on the site. He loves helping fellow aquarists and introducing new people to the hobby!

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