The 17 Most Common Molly Fish Diseases & How to Treat Them

The 17 Most Common Molly Fish Diseases & How to Treat Them

Mollies are known for being tough freshwater fish that can tolerate a wide range of water conditions. 

However, they are still susceptible to a number of diseases that can be fatal if not treated promptly.

Many diseases can affect molly fish, like ich, and marine velvet, as well as fungal and bacterial infections.

In this article, we will discuss some of the most common diseases that molly fish can get, as well as how to treat them.

What Are Molly’s Fish Diseases & How to Treat Them?

1. Gill & Fin Flukes

Flukes are a type of parasitic infection in which a tiny white worm burrows itself into the fish’s fins, gills, and body and drains its blood, resulting in breathing problems and bleeding.

Therefore, you can see mollies with flukes gasping for air on the surface of the tank or at the bottom of the aquarium.

Since these tiny worms may be inadvertently introduced into the aquarium via new plants or new fish, you should quarantine all new fish and clean any new plants carefully before adding them to the molly fish tank.


In its early phases, there are many different gill fluke basic medications to try, and you should clean your tank regularly.

However, in more advanced situations, such as when bleeding has already started, no therapy is available.

2. Fish Fungus

Fish fungus is a highly contagious bacterial infection. It manifests as white or gray patches on your Molly fish’s body, fins, and scales that soon spread to their eyes.


If your mollies have fungus, you should visit a veterinarian since the treatment for this condition necessitates the usage of particular antibiotics.

3. Swollen Gills

Swollen gills are caused by an accumulation of excess mucus on your mollies’ gills, which prevents them from breathing normally.

Ammonia or carbonate poisoning can cause this excess mucus production since both irritate your mollies’ gills.


  • Perform a 50% water change and monitor toxin levels over the next few days and clean the tank regularly
  • Don’t give your mollies food for a couple of days after the swelling subsides to avoid adding more waste to the water
  • Add some nitrifying bacteria to the water (you may use API Quick Start)

4. Fin and Tail Rot

Fin rot is one of the molly fish diseases caused by bacteria. It begins with a minor portion of the fins damaged. Then it progresses gradually until the disease affects almost the entire tail.

In more advanced phases, the tail will be chewed up, shredded, or stitched together, as well as white milky areas in other regions of the body will start to appear, while the fin will be ragged.


  • Separate and quarantine the sick fish in a hospital tank
  • Begin on antibiotics for bacterial infections (antibiotics that target gram-negative bacteria such as Maracyn, Maracyn 2, and Tetracycline). 
  • Perform 20-50% water change in the origin tank

5. Dropsy Disease

Dropsy disease is caused by parasites in a fish’s gills or digestive system releasing poisons into the body, causing fluid to build up beneath the scales. 

It causes kidney and liver ailments that lead to water retention.

Because of water retention, molly fish will appear bloated with scales sticking out, protruding eyes, and unable to swim.


Dropsy is difficult to cure, and in most situations, it’s already too late for the fish to be saved once they begin showing symptoms.

I recommend Epsom salt baths, however, they will only slow down the process or make your fish more at ease.

6. Swim Bladder Disease

The swim bladder is a gas-filled internal organ that assists fish in maintaining neutral buoyancy, balances them while swimming, and keeps them upright in the water.

So, when this organ is infected or inflamed, it will push the fish’s intestines into its abdomen, which causes a bloated look.

Mollies with swim bladder problems will often have trouble swimming and may even float upside down or lay at the bottom of the tank.

Swim bladder disorder is caused by a virus and is untreatable. Therefore, mollies with swim bladder disease should be euthanized as soon as possible and ethically disposed of.

7. Fish Lice

Fish lice are tiny white dots that crawl all over your mollies’ bodies. They are highly transmissible and can be transferred from one fish to another during feeding or by touching.

Lice are frequently transferred between sick fish by human hands. So, if you ever handle a sick fish, you should do so carefully to avoid spreading these infections.


  • Quarantine the sick fish in a separate tank
  • Perform regular water changes at a rate of 25-50%
  • Avoid adding too many fish to your tank so you can spot the ill molly early and treat it with an insecticide.

8. Ich

White spot disease (ich) is one of the most common molly fish diseases and it is caused by an ectoparasite.

It’s easy to detect because of the white patches that form on the skin and fins of your molly fish.

Aside from the spots that appear on your molly fish, you’ll see them rubbing against things in the tank in an attempt to scratch the spots. 

In addition, the disease may cause your fish to lose their appetite.


  • Move the infected fish to a hospital tank to avoid infecting the other fish in the community tank
  • Raise the temperature in the tank gradually to 80 F
  • Add Seachem ParaGuard at the recommended dosage (see label) or aquarium salt (1 teaspoon/gallon).
  • Keep the temperature and treatment up for 4-7 days
  • Perform a significant water change (70%) and make sure everything is thoroughly cleaned.

9. Velvet

Velvet disease is caused by an aquatic parasite known as Oodinium, which burrows into your fish’s skin and creates small, golden-colored cysts. This illness spreads rapidly and frequently has deadly effects.

It’s critical to act as soon as possible in the early phases when the cysts are tiny. If you wait until lesions appear on the skin, you risk having a lot of dead fish.


  • Copper medicines have excellent effects, such as Seachem Cupramine.
  • For the treatment to be effective, you must first turn off the aquarium lights for the duration of time and until the disease is gone.
  • Once disease symptoms have disappeared, perform a large water change (70%-90%).

10. Protozoan

Protozoa parasites thrive in poor water quality and can quickly enter your molly fish’s bloodstream via wounds or abrasions.

They can also attach themselves to the skin and slowly insinuate themselves into the muscles until they reach the bloodstream.

When a molly fish is infected with this disease, it will have tiny white flecks on its skin, excessive slime, an inability to eat, and listlessness.


11. Mouth Fungus & Columnaris

Mouth fungus is a bacterial infection characterized by tiny white strands or splatters on the mouth or body of fish.

It can be accompanied by ragged fins, fast breathing, excessive mucus production, and wounds and ulcers in severe situations.


  • Antibiotic treatment with Maracyn or Formalin
  • Add aquarium salt to the aquarium (one teaspoon per gallon) every day for 3 days
  • After the symptoms have faded, perform a 50-70% water change.
  • You may also bathe your fish in a solution of 10 mg/l potassium permanganate for 30 minutes. However, be cautious not to exceed the dosage and treatment duration since you may risk burning your fish with this chemical.

12. Red Blood Spot

Red blood spots are frequently observed in an aquarium that has not completed the nitrogen cycle and has incorrect water chemistry.

This illness is incurable and ultimately leads to your fish’s death. Therefore, you should get to know the aquarium cycle and wait at least 2 weeks before adding mollies to your community tank.

13. Viral Haemorrhagic Septicaemia

This is a viral disease that causes sores, ulcers, pale gills, and bulging eyes and can eventually result in fin rot.

Infected fish will stop eating, changing of molly fish color, and ultimately die as the illness progresses.


  • Use antibiotics as soon after the disease appears as possible (Maracyn 2 or API Furan 2 are effective against VHS)
  • Perform a major water change after treatment

14. Camallanus Parasite

This parasite is green or orange in color, and it can be seen protruding from a fish’s anus.

The disease may be spread throughout your aquarium by new fish, particularly those that have previously lived in ponds.


  • This disease is treated with a 5-day treatment course that includes Levamisole (Ergamisol) or Fenbendazole and Parcide X or D
  • After treatment, perform a significant water change (90 percent or more) and vacuum the substrate.
  • Repeat the process in 3 weeks.

15. Fish Tuberculosis

Tuberculosis is a disease transmitted by mycobacterium bacteria. 

A molly fish afflicted with tuberculosis will have an aversion to food, ulcers on the anus and body, fading colors, and rotting fins and tails.

Unfortunately, fish TB is incurable and can be transferred to humans. Therefore, get rid of sick fish and their dead bodies as soon as possible.

16. Bent Spine

Scoliosis, or bent spine deformity, causes fish to have difficulty swimming owing to their twisted backs as a result of poor genetics.

Molly fish with scoliosis may struggle to grow properly, be less robust than other offspring, be harassed by other fish, and have a shorter life span.

Because it is a consequence of poor genes, bent spine illness cannot be cured. 

However, it can be prevented by not allowing fish with this problem to breed and ensuring that you’re producing healthy adults.

17. Hexamitiasis

The Hexamitia parasite is not very common among mollies, but it can infect molly fish in rare circumstances.

The color of mollies infected with this parasite fades, tumefied areas appear on the head or body, they lose interest in eating, and they have difficulties swimming.


  • Use medicated food (metronidazole, brand name Flagyl)
  • Treat the water with 250 mg per 10 gallons once a day for at least three days.

How to Keep Molly Fish Healthy?

1. Maintain High Water Quality

One of the important things you can do to keep your molly fish healthy is to maintain high water quality in their aquarium.

Therefore, you should do a water change of at least 25% once a week and vacuum the gravel to remove debris and uneaten food that can cause ammonia spikes.

It’s also vital to monitor your water on a regular basis by using an ammonia, nitrite, and nitrate test kit to ensure that these levels are within acceptable limits.

In addition, an aquarium filter will help to keep your water clean and provide a place for beneficial bacteria to grow.

2. Adjust Proper pH Level & Water Temperature

Water parameters play a great role in preserving the health of molly fish.

The ideal water temperature for mollies is between 72 and 78 degrees Fahrenheit, while the ideal pH level is between 7.5 and 8.5.

To make sure you are maintaining a pH level within the acceptable range, you can use a pH test kit. And to ensure the water temperature remains stable, use an aquarium heater.

3. Add Hiding Places & Live Plants

Hiding places are essential for mollies as they help them to reduce stress levels and provide a place to escape from aggressive fish.

This is why you should provide your fish with hiding places such as caves, driftwood, and rocks.

You should also add live plants to your tank as they provide oxygen and help to absorb harmful toxins.

Aside from providing oxygen, live plants also help make your aquarium more aesthetically pleasing as they come in a wide variety of colors and sizes.

4. Provide Them With a Proper Diet & Feeding Schedule

Mollies are omnivores, so they require an equal amount of plant and animal-based foods in their diet.

As a result, be sure to feed mollies plenty of veggies like cucumbers and zucchini. These nutrients will aid your fish in battling parasite-related illnesses.

You should also feed them tiny portions of food twice a day and never leave food remains in the aquarium since this may pollute the water.

What Makes Molly Fish Prone to Diseases?

Like all fish, mollies are susceptible to a variety of diseases that can make them sick or even kill them. Some of the most common reasons molly fish fall ill are:

  • Improper temperature
  • Direct contact with other fish that are sick
  • Poor water quality
  • Aggressive tank mates


What Are The Signs That Your Molly Fish Is Sick?

These are a few signs that your molly fish is sick:

  • It has a dark patch on its body
  • Molly fish acts sluggish
  • When it starts swimming in circles or in tight patterns
  • Lack of appetite

Is Keeping Molly Fish Alive Difficult?

The majority of these species are hardy and easy to maintain, making them perfect for novices.

They’re an excellent choice for quiet social tanks, and if maintained correctly, a group of mollies can live up to five years in captivity.

Do Sick Molly Fish Need Aquarium Salt?

Molly fish are tropical freshwater fish. So, it would be beneficial if you dusted them with salt while they were ill.

This is due to the fact that salt aids in the healing of fish wounds. It can also kill germs that cause disease in fish.

However, aquarium salt can kill beneficial bacteria, so you should consult your vet first.

Last Words

The best way to prevent your molly fish from getting sick is to maintain a clean and stable environment. You should also provide them with a proper diet and feeding schedule.

By following the tips in this article, you can help to keep your molly fish healthy and reduce the risk of them becoming sick.

We hope you enjoyed reading this article and found it informative. If you have any questions or comments, please feel free to leave them below. 

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