15 Common Pet Fish Diseases: Symptoms and Treatments

Noticing something off with your fish? Quickly identifying and treating common pet fish diseases is crucial for their well-being and survival.

Mar 27, 2024By Monika Dimitrovska
common pet fish diseases symptoms treatments

Common pet fish diseases come from poor water quality, stress, overcrowding, and not isolating new or ill fish to stop disease spread. However, you can minimize these factors with proper care and good hygiene.

Pet fish can also experience infections caused by viruses, bacteria, parasites, protozoa, and fungi. In this article, we’ll talk about common diseases in fish, symptoms, treatment, and prevention, so let’s jump right in!

Most Common Pet Fish Diseases and Treatment

sick fish
Image credit: aquariadise.com

Fish make fantastic pets, but like all living beings, they get sick.

Unfortunately for fish owners, identifying the symptoms of common pet fish diseases can be hard because they present subtly and can worsen without prompt treatment.

That being said, here’s a list of common fish illnesses and disorders, along with their symptoms and possible treatment:

1. Ammonia Poisoning

ammonia poisoning fish
Image credit: aquariadise.com

In new fish tanks, ammonia poisoning is common. It’s not a disease, but it can make your fish sick. Ammonia comes from breaking down organic matter. Bacteria change it into nitrite and then into nitrate, which is less harmful.


  • Hard time breathing
  • Gasping at the surface
  • Not eating much
  • Feeling lazy
  • Red streaks on the body
  • Gills looking red or swollen
  • Hanging out at the tank bottom


Make sure a new tank has finished the nitrogen cycle and is packed with helpful bacteria before adding fish. Keep ammonia spikes in check by changing the water a lot. Also, when changing water, use a siphon to clean out gunk from the bottom.

Proper care and tank maintenance can help fish owners live in harmony with freshwater fish.

2. Anchor Worms

anchor worms fish
Image credit: Wikipedia

Anchor worms or parasitic crustaceans often show up in pond fish. However, they can also infect aquarium fish. They dig into muscles or even internal organs, forming ulcers at their attachment points, which might lead to more infections.

The good news? You don’t need a guide on basic fish anatomy to identify the following signs of anchor worms:


  • Red, inflamed skin
  • Visible worms
  • Ulcers
  • Rubbing against tank objects


Anchor worms need quarantine. Using forceps, manually remove adult worms and treat the wound with a fish-safe antiseptic. Keep at it weekly until no more adult worms pop up.

3. Brooklynellosis (Anemonefish Disease)

anemonefish disease clownfish
Image credit: Sarah Peets from Pixabay

Caused by protozoa called Brooklynella hostilis, this disease often hits anemonefish but can affect others, especially when they’re stressed, like in crowded stores or during shipping.


  • Skin sloughing
  • Extra mucus
  • Laziness
  • Breathing trouble
  • Loss of appetite
  • Discolored skin


Treat with a mix of formalin and malachite green, dosing at 0.10 ppm for 7 to 10 days. Formalin, at 0.5 mL per gallon, is best for baths. Be cautious with damaged skin fish; they might be more sensitive.

4. Columnaris (Mouth Fungus)

mouth fungus fish
Image credit: plantedtank.net

Also called cottonmouth or mouth fungus, Columnaris is caused by Columnaris bacteria, showing as white or gray spots on the head, fins, and gills that turn into cottony growths.


  • Cottony growths
  • Raised skin patches
  • Pale gills
  • Laziness
  • Loss of appetite
  • Swimming problems


Injectable antibiotics work best. External treatment may help early on, according to a 2013 study.

5. Dropsy

dropsy fish
Image credit: fishkeepingfans.com

Dropsy, seen as a swollen abdomen and scales sticking out, is a symptom of disease, often caused by opportunistic bacterial infections.


  • Swollen abdomen
  • Protruding scales


Antibacterial meds work for bacterial infections. Metronidazole might help with protozoan infections, but viral causes pose a greater challenge.

6. Fin Rot

fin rot fish disease
Image credit: fishlab.com

Fins or tails rotting or fraying is fin rot, often linked to poor water quality and sometimes worsened by fish nipping or bullying.


  • Changing the color of the fins or tail
  • Frayed edges
  • Inflammation at the base
  • Deterioration of fins or tail


Fix water quality; sometimes, bacterial infection requires antibiotics.

7. Flukes

Microscopic parasites like Gyrodactylus (skin flukes) or Dactylogyrus (gill flukes) can irritate a fish’s gills or skin. Even though they’re too small to see, you can spot them through the irritation they cause.


  • Skin irritation
  • Missing scales
  • Red spots
  • Extra mucus
  • Flashing
  • Laziness
  • Loss of appetite


Flukes often come from poor water quality, overcrowding, or a bad diet. Proper identification is crucial for flukes treatment. Formalin is common, but use it carefully; high doses can reduce oxygen in the water and be toxic.

8. Gold Dust Disease (Velvet)

gold dust disease fish
Image credit: thesprucepets.com

Caused by a protozoan parasite attacking the skin and gills, velvet or gold dust disease has a dusty yellow appearance. Different parasites cause it in freshwater and marine fish.


  • Dusty yellow appearance
  • Tiny spots on fins and body
  • Flashing against surfaces
  • Loss of appetite
  • Laziness


Vets usually recommend chloroquine for treating velvet in aquarium fish. Follow instructions carefully and check for symptoms returning after 7 to 10 days.

9. Hexamitiasis

hexamita betta fish
Image credit: ivabalk from Pixabay

Hexamita, a protozoan parasite, often affects cichlids, gourami, and betta fish. They attack the intestines, causing weight loss and, in severe cases, death, especially in already-stressed fish.


  • Weight loss
  • Loss of appetite
  • White, stringy feces
  • Death


Quick hexamita treatment is crucial to prevent serious weight loss and death. If the fish is still eating, use metronidazole in food. If not, add 250 mg per 10 gallons to the water.

10. “Hole in the Head”

hole in the head disease butterfly cichlid
Image credit: Sunny from Pixabay

This disease affects perciform fish like cichlids and carp. Hexamita parasites, usually intestinal, can move to the fish’s head, causing tissue decay and deep lesions.


  • Pale-colored feces
  • Lesions on the head
  • Tissue decay


Maintaining top-notch water quality is crucial to prevent the hole in the head disease. If it develops, treat it with metronidazole-medicated food. In severe cases where fish stop eating, add the medication to the water.

11. Ich (White Spot Disease)

white spot disease fish
Image credit: jbl.de

One of the most contagious diseases in freshwater tank cleaners and other fish, ich or white spot disease, comes from the protozoan parasite Ichthyophthirius multifiliis.

It forms tiny cysts on the fish’s body, fins, and gills, initiating strange fish behaviors like hiding.


  • Tiny white spots
  • Dusty skin appearance
  • Loss of appetite
  • Laziness
  • Labored breathing
  • Hiding behavior


Quarantine sick fish; raising the tank temperature speeds up the parasite life cycle, resolving the infestation quicker. Recommended treatments include copper sulfate and malachite green.

12. Lymphocystis

fish swimming lymphocystis
Image credit: Rudy and Peter Skitterians from Pixabay

Caused by a viral infection, lymphocystis shows raspberry-like growths, often due to poor water quality, but can also be inherited or transmitted through contact.


  • Raspberry-like growths
  • Popeye


There is no cure for viral infections, but if tumors affect eating or swimming, surgical removal may be needed. Otherwise, it usually gets better by itself in a few weeks.

13. Marine White Spot Disease

marine white spot disease fish
Image credit: fishkeeper.co.uk

Marine white spot disease is similar to freshwater ich but caused by Cryptocaryon irritans (ciliated protozoan with a three-stage life cycle). You can recognize this disease in colorful pet fish species as their color fades. Other signs include:


  • White spots on the body and fins
  • Dusty appearance
  • Labored breathing
  • Flashing against substrate


Quarantine the affected fish; frequent water changes reduce parasites. Formalin or copper-based meds kill parasites. Lowering salinity to 14-16 ppt can halt the parasite’s life cycle.

14. Parasite Infections

parasite infections fish
Image credit: fishkeepingfans.com

Common in stressed fish, especially in less-than-ideal conditions like fisheries or retail stores. Examples of parasite infections in fish may include roundworms, epistylis, and flatworms.


  • Visible dots or threads
  • Inflamed gills
  • Labored breathing
  • Flashing


Manual removal for larger parasites; quarantine sick fish to prevent spreading. Formalin or praziquantel treatments may help in some cases.

15. Popeye

popeye fish
Image credit: Wikipedia

Popeye is swelling of one or both eyes due to fluid buildup caused by a bacterial infection caused by stress or poor tank conditions.


  • Swollen eye
  • Cloudy eye


Swelling may reduce if tank conditions improve, so make sure you set up your tank properly. Experts suggest treating a hospital tank with aquarium salt.

Final Tips

fish swimming
Image credit: Pexels from Pixabay

In conclusion, keeping your fish healthy boils down to a few key practices.

Ensure a well-balanced freshwater aquarium and proper diet, and observe your fish regularly for signs of disease. Additionally, when introducing new members, quarantine them first to prevent potential issues, especially if they’re fish you caught yourself.

By following these simple steps, you’re not just caring for your fish – you’re creating a safe and thriving aquatic home.


Q: How can pet owners differentiate between symptoms of similar diseases, such as Ich and Marine White Spot Disease, to ensure the correct treatment is applied?
A: Differentiating diseases like Ich and Marine White Spot involves observing the disease's progression and specific symptoms, such as spot size and fish behavior. Consultation with a vet or experienced aquarist can provide accurate diagnosis and treatment guidance.

Q: What specific diet recommendations can help prevent common fish diseases or support recovery during treatment, beyond the general advice of a proper diet?
A: Specific diet recommendations include foods rich in vitamins and minerals, such as vitamin C and E, to boost the immune system. Offering a variety of foods, including live, frozen, and pellets, ensures balanced nutrition.

Q: Are there any natural or holistic treatments or preventative measures for common fish diseases that can be used as alternatives or supplements to chemical treatments?
A: Natural treatments include maintaining optimal water quality, using salt baths for certain conditions, and incorporating immune-boosting supplements into the diet. However, these should complement, not replace, advice from a veterinarian.

Monika Dimitrovska
By Monika Dimitrovska

Monika is a pet enthusiast and seasoned copywriter with a tech degree. She loves writing, but her heart belongs to her two mixed dogs, Buba and Bono, a mother-son duo. Bono’s siblings found loving homes, sparking Monika’s advocacy for neutering and deepening her curiosity about animal care.

But Monika’s pet family doesn’t end there. She also has two cockatiels and two rescue cats, proving her home is a haven for creatures big and small.