Fish diseases are due to stress, poor living environment in the aquarium, and when a new fish does not undergo quarantine standards. However, measures like frequently changing your fish aquarium water and feeding your fish pet healthy meals can help prevent some fish illnesses.

It is also crucial to isolate sickly fish immediately to observe any symptoms to avoid an outbreak.

Read this article to learn about the different diseases marine and freshwater fishes are susceptible to develop.

Aquarium Fish Diseases

Catfish swimming at the bottom of aquarium due to fish disease

Aquarium fish diseases are disease types that tank-bred fishes are prone to get.

We have categorized into two:

  • Freshwater fish diseases
  • Saltwater fish diseases
  1. Freshwater fish diseases: these are diseases that freshwater fishes are likely to get. Some of them are skin and gill flukes, dropsy, Tetrahymena, vibriosis, Anchor worm, mouth fungus (columnaris), fin rot, pseudomoniasis, swim bladder diseases, saprolegnia fungus, piscine tuberculosis, Neon tetra disease, ich, iridovirus, lymphocystis, Singapore Angelfish diseases, hole-in-the-head, Malawi bloat, and many more.
  2. Saltwater fish diseases: these are diseases that Salt, marine, or euryhaline fishes are prone to contact. Some diseases are Anemonefish Disease, Marine Velvet or Coral Reef Fish Disease, and Cryptocaryon (marine ich).

However, some illnesses are common between the two fish types: velvet disease, marine velvet, and Septicemia or Egtved virus.

Causes of Fish Diseases

Fish tank diseases are classified under different primary diseases causing agents.

They are:

– Fish Parasitic Infections

They are highly infectious and are transferred by a parasite to a host. The host is the fish, while the parasite feeds on it until it can complete its life cycle. In most cases, it is fatal for both the host and the parasite.

Various kinds of aquarium fish parasite infections include:

  • Protozoan parasites like ich, velvet, hole in the head, and neon tetra disease
  • The external arthropod parasite (such as anchor worm and fish louse)
  • Trueworm parasite/nematode (such as fluke and roundworm)

1. Protozoan Parasitic Infections

a) Ich


It is caused by Ichthyophthirius multifiliis in freshwater fish and Cryptocaryon irritans for Marine fish. A stressed fish in a non-conducive environment is prone to ich.


  • The fish body looks like it is covered in salt granules
  • It would try to scratch its body on any object in the aquarium
  • Its fins would be clamped
  • It would have challenges breathing, particularly when the ich infection reaches the gills

(Ich) Fish parasite Treatment

  • Administer antibiotics to the fish
  • Do not give scaleless fishes a large dosage of malachite green
  • Marine fishes can recover from ich with garlic treatment
  • It may reoccur in the fish if you treat for less than seven days
  • Clean the aquarium because the cysts may be present at the bottom of the tank

b) Velvet

It is caused by Oodinium limneticum in freshwater fish and marine fishes; oodinium ocellatum is the culprit.

Symptoms of Velvet

  • It’s similar to whitish particle-like, but it is smaller and gives the illusion of a velvety finish
  • The fish would be itchy and scratches itself on different objects
  • It clamps down its fins

Treatment Of Velvet

Antibiotics work to tackle the fish disease.

c) Hole-in-the-Head Disease

It is caused by Hexamita sp. and Spironucles sp. However, some fish-keeping enthusiasts would argue about the cause.

Symptoms of Hole-in-the-Head Sickness

  • Wide holes would show on the head of the fish
  • In some fishes, the lateral line would erode due to poor water quality
  • The lesions may bring out pus
  • The fish would appear lethargic
  • Would have trouble finding its balance


You may use antibiotics like metronidazole.

d) Neon Tetra Disease

Neon tetra fish disease

It is caused by “Pleistophora hyphessobryconis.” It is highly contagious and can affect danios fish species.

Symptom of Neon Tetra Disease

  • There is loss of coloration in the milk, brown, or red body-color
  • It becomes lethargic and swims erratically
  • It becomes withdrawn and always wants to be alone
  • It looks wasted and lean
  • In severe cases, it develops pop-eye and stomach bloating


No documented cure. However, try to keep the sick fish comfortable by performing water changes and feeding it well.

2. External Arthropod Parasite

a) Anchor worm


The lernaea spp causes the anchor worm. The lernaea parasites are mini-crustaceans widely spread in fresh or saltwater, and they may be benthic or planktonic. It attaches itself to the fish mouth using its whip-like tail.

Symptoms of Anchor Worm

There is usually no change in the fishes’ behavior except in extreme conditions like another fish infection.


The fish disease treatment is not complicated as you need to detach the anchor worm from its host. It will lead to intense pain and discomfort and should be done with care. Use a bit of antibiotic (such as common brand name aquarium medication) to rub the injury’s surface.

b) Fish Louse

The Argulus app causes the fish louse. It is a flat oval-shaped parasitic agent with two eyes, many legs, a sucking and piercing mouth, and a suction body part that helps it attach to a host.

Symptoms of fish louse

The fish would not exhibit any obvious different behavioral trait but may behave strangely in severe cases.


To treat the fish louse infestation on the fish, inspect the fish body and take out the parasite. It would cause discomfort because the louse has firmly latched to the fish’s body, so do it slowly and with great care. Clean the site of the wound with antibiotics when you have removed it.

3. True Worm Parasites/Nematodes

a) Fish Flukes

It commonly occurs on the gills, skin, and eyes of the fish. Dactylogyrus vastator causes fish gill fluke while the skin fluke is primarily caused by Gyrodactylus sp.

Symptoms of Fish Flukes

  • The gills of fish would look reddish and inflamed
  • The fish would have irritated skins and would be scratching its body against surfaces
  • The fish would have challenges breathing

Treatment of Fish Fluke

  • Frequent water changes and antibiotics help
  • Quarantine infected fish

b) Roundworms

They are a rare nematode infection that causes fish sickness.

Symptoms of Roundworm

  • It affects the connective muscle tissues and the gut of a fish
  • The fish loses appetite and appears lethargic

Treatment of Roundworm

Anti-parasitic medicine would cure roundworm infections

– Bacteria Infection

Below are some bacterial fish diseases.

a) Bacterial Fin Rot

Fish suffering of bacterial fin rotDifferent organisms cause it.

Symptoms of Bacterial Fin Rot

  • The fish’s fin appears shredded, tattered in some cases, and affects the fish pedicle
  • The fish also has challenges swimming around


  • You should administer a broad spectrum antibiotic
  • Change the water in the aquarium often and test for nitrates

b) Pop-Eye

Fish pop eye diseaseIt is not a disease, but it is a symptom of various illnesses and severe stress conditions. However, some of its causes are curable, while others are incurable. It is not infectious but can lead to death, loss of sight, or popped/missing eyeball.

Secondary Infections That Lead To Pop-Eye are parasitic fish fungus like “Ichthyophonus, Ichthyosporidium,” fish tuberculosis, and worm cataract disease.

Symptoms of Pop-Eye

  • The eyes of the would look so bulgy like it is about to pop from its socket
  • Pus and excess fluid content in the fish eyes
  • In severe cases, the fish would lose its sense of sight
  • It may behave lethargically


  • Change the aquarium water frequently to reduce ammonia and nitrates
  • Treat with a broad spectrum of antibiotics (such as tetracycline) directly to the secondary infection that led to pop-eye symptoms
  • Administer Terramycin to the infected fish for ten days
  • If the fish eyes have fluke infestation, use malachite green with either copper, metriphonate, or formalin

c) Cloudy Eye

This aquarium fish disease is caused by malnutrition, cataract, poor water environment, excess slime production in the aquarium, old age, and stress.



  • Feed the fish healthy meals
  • Change the dirty aquarium water often and test for nitrates
  • Use some antibiotics

d) Mouth Fungus/ Columnaris

The Mycobacterium spp causes it.

Symptom of columnaris

  • The fish tank disease causes inflamed patches of white and pink to appear on the fish mouth, with some parts looking fuzzy
  • It also has ulcers on its head and body
  • It would constantly be hiding
  • It would be lethargic and have little appetite
  • The fish would clamp its fins


  • Administer some antibiotics to the fish, but it may be challenging to treat the fish sickness
  • Change the water in its aquarium often
  • Test the water from ammonia often

e) Fish Tuberculosis


It is caused primarily by bacteria like M. piscium Mycobacterium spp. and M. marinum.

Symptoms of Fish Tuberculosis

  • This aquarium fish disease is difficult to diagnose, unless you perform a lab culture with a fish sample
  • It looks sick and lean with its stomach shrunken
  • It develops a curvature on its spine in severe cases
  • It has no appetite and vomits after feeding
  • You find your fish constantly hiding
  • It is lethargic and clamps its fins down


  • Fish tuberculosis attacks the cells of the fish and other multiples in the macrophages.
  • Administer streptomycin or other strong antibiotics. You can use Euthanasia in extreme conditions.

f) Dropsy Fish Disease


It is caused by poor water conditions and other organisms present in the aquarium. However, it is not contagious.

Symptoms of Dropsy

  • The fish appears so bloated that its scales do not lay flat on its skin
  • Excess fluid would accumulate in the fish body
  • In severe cases, the fish experiences organ failure
  • The fish would behave sluggishly and have no appetite to eat

Treatment of Dropsy

  • Drop by has no cure, but in some cases, the fish would suddenly begin to recover
  • Perform a frequent water change
  • Use strong antibiotic like kanamycin sulfate

g) Hemorrhagic Septicemia

It is caused by a sudden increase of ammonia in the aquarium water.

Symptom of Hemorrhagic Septicemia

  • A huge red marking would appear on the fish fin
  • The Mark is a result of bacteria fish infection in the vascular system of the fish
  • Red patches on the fish Body
  • It would have challenges breathing as it shows signs of gasping
  • The fish would appear restless, and you see it swimming in an erratic manner

Treatment of Hemorrhagic Septicemia

  • These fish diseases tend to go away by themselves some times as the water quality improves.
  • You should treat it with antibiotic medication.
  • If the fish exhibits other symptoms associated with excess ammonia in the water, test the water’s ammonia levels.

h) Enteric Red Mouth

The “Yersinia ruckerii” causes it. The fish disease is common with wild fishes yet rate in tank-bred ones.

Symptoms of Enteric Red Mouth

  • The fish mouth and belly would appear red
  • Internally, the liver of the fish may be attacked by the parasite
  • The fish would appear sluggish and lethargic
  • It would lack the appetite to eat

Treatment of Enteric Red Mouth

To treat this tropical fish disease, reduce the water temperature in the tank.

i) Swim Bladder Disease

It is caused by various bacteria infections, stress, indigestion, and failure to acclimate in the new tank.

Symptom of swim bladder disease

  • Indigestion in some fishes
  • You may notice slight bloating in your fish
  • The fish may seem to be swimming in a strange manner with its belly facing upwards

Treatment of Swim Bladder Disease

You can treat the swim bladder disease in the goldfish by temporarily stopping their feeding. Then utilize a salt bath with a sudden diet change to high fiber meals. Administer antibiotics to the fish in worse scenarios, but there is no particular cure to the fish disease

– Fungal Infection

We have the body fungus and cotton fin fungus.

1. Body Fungus

It is caused by Saprolegnia spp. The body fungus causes a white or grey fuzzy-like skin or fin growth similar to the mold that grows on decaying food. Some fish-keeper believes the body fungus looks like a columnaris infection. However, body fungus grows on dead cells or tissue, and bacterial infections can thrive in fungus on the same site. They affect stressed, weak fishes in poor living conditions.

Symptom of Body Fungus

There is no obvious body growth until when the infection is severe.

Treatment of Body Fungus

Antibiotic with methylene blue in its composition

2. Cotton Fin Fungus

It is caused by Saprolegnia spp.

Symptoms of Cotton Fin Fungus

  • The fungal growth looks like cotton; it appears soft and fuzzy. However, it causes dead tissue to rot.
  • The fish do not exhibit strange behavior until the advanced stages of cotton fin ro


  • Medications that contain methylene blue
  • Perform water changes often and intensify the system circulation because the causative agent thrives in the stagnant water body
  • In most cases, the fish recovers from cotton fin rot when the water conditions improve

– Viral Infections

They include tumors and lymphocytic.

1. Cauliflower Disease or Lymphocystis

The cauliflower fish disease is caused by Lymphocystis sop or iridovirus. The infection causes the overproduction of red blood cells leading to growths and tumors. They are obvious on the pedicle, lateral line, and at the point, the fish fin connects to the body. The growth looks like cauliflower. It infects big fishes that eat live meals, especially cichlids.

Symptoms of Cauliflower Disease

  • They become lethargic
  • They lose their balance, and swimming becomes erratic

Treatment of cauliflower disease

  • Change the aquarium water often to get rid of excess ammonia, nitrite, and nitrate.
  • A conducive water environment would help the fish heal from the infection, and the tumors would go away gradually.

2. Unknown Tumor

It is caused by a viral infection that switches off genes responsible for tumor suppression. It can be cancerous or benign; however, aquarists do not thoroughly understand it.

Treatment of tumor

There is no known treatment for this type of tumor. However, surgical removal is the only option. Visit a certified veterinary doctor to help you with the procedure. In rare cases, the tumor shrinks without any medication. You can try Euthanasia as a treatment route for tumor growth.


  • Goldfish turned upside down due to aquarium fish diseaseFishes are susceptible to illness due to poor living conditions
  • You can treat most diseases with broad-spectrum antibiotics
  • To prevent aquarium fish disease, monitor the fish, so it is not stressed and feeds properly
  • It is crucial to know that some drugs’ side effects are more intense than the symptoms of the illness you want to treat

Some fish disease treatment can disrupt the nitrogen cycle by killing off all the good bacteria in the fish tank. Consider the aquarium plants as you administer treatments because a few are very sensitive.

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