Molly Fish Diseases, Parasites & Remedies

It’s extremely disheartening to have a pet fall ill, especially if you don’t know what’s wrong with it and what you can do to help. Unfortunately, like most animals, molly fish can get sick too.

Knowing the diseases that can affect mollies and knowing how to prevent and treat these diseases can help you better manage situations when problems appear.

Most common molly fish diseases such as ich, fin rot, flukes or velvet can be treated with the Seachem ParaGuard medication.

This fish medication is very effective, and can be purchased from Amazon.

In what follows, you can read about the most common molly fish diseases, parasites, and remedies. I’ll also give you some useful and practical tips on molly fish disease prevention.


I hope that by having a clear overview of these diseases, their symptomatology and possible remedies, you’ll know what to do if you come across any of the diseases presented in this article.

Most Common Molly Fish Diseases

These are the diseases and illnesses you’re most likely to encounter in a molly fish tank, so take notice of their symptoms and treatment.

1. White Spots (Ich or Ick)


Molly Fish Infested with Ich (White Spots)

Easy to notice because of the white spots that appear on the skin and fins of your molly fish, Ich or Ick disease is easy to cure if you start treatment on time. The disease is caused by an ectoparasite.

Besides the spots that appear on your fish, you’ll also notice them rubbing themselves against objects in the tank in an attempt to scratch off the spots. The disease may be accompanied by loss of appetite as well.

Treatment of White Spots:

  • Raising temperature in the tank slowly to 80 F;
  • Adding Seachem ParaGuard into the tank in the recommended dose (see label);
  • Alternatively, adding aquarium salt (1 teaspoon/gallon).

Keep the temperature and treatment up for 4-7 days, then perform a major water change (70%), making sure you thoroughly clean the substrate too.

Medication should be applied on a tank level, even if you have a separate hospital tank, where you treat infected fish.

2. Velvet


Molly Fish with Velvet Disease

Velvet disease is caused by the parasite Oodinium, which burrows itself into the skin of your fish causing small gold-colored cysts. This disease spreads quickly and often with devastating consequences.

Immediate action is crucial to save your molly fish, especially that detecting the disease in its early stages in difficult as the cysts are very small at first. Only when the disease progresses do lesions start appearing on the skin.

Treatment of Velvet disease:

  • In its early stages, copper medication shows great results in treating velvet (I go with Seachem Cupramin);
  • Switching off the aquarium lights for the duration of the treatment and until the disease is eliminated also helps;
  • A major water change (70%-90%) once disease symptomatology can no longer be detected is also necessary.

3. Fin and Tail Rot

Fungal infections or bacterial infections can both cause molly fish fin rot disease. The proliferation of bacteria and fungus in the tank as a result of ammonia spikes and poor tank conditions are the main culprits for the disease.

If your fish are injured while tank conditions are bad, opportunistic bacteria and fungi will take advantage of it.

A tail that looks like it has been chewed up, shredded or stuck together, along with white milky regions in other parts of the body (advanced stages) are all signs of fin and tail rot.

Treatment for fin and tail rot:

  • Remove and quarantine infected fish;
  • Start them on antibiotics for bacterial infections (antibiotics that target gram negative bacteria like Maracyn, Maracyn 2, Tetracycline; alternatively, you can also use Seachem ParaGuard);
  • Use fungal medication for fungal infections;
  • Perform 20-50% water change in origin tank.

Be advised that damaged fins or tails will not regrow.

4. Protozoan

Protozoa thrive in bad water conditions and can easily enter the bloodstream of your fish via cuts, scrapes or by simply attaching itself to the skin and slowly getting itself into the muscles until it eventually finds its way into the bloodstream.

The disease can be identified by small white specks on skin, excess slime, loss of appetite, lethargy.

Treatment of protozoan disease:

  • Keeping water temperature stable (this is actually one way to prevent the disease, get an aquarium heater with a thermostat);
  • Malachite Green or Formalin in early stages, copper medicine in advanced stages;
  • 50-70% water change after following through with the treatment.

5. Mouth Fungus & Columnaris


Molly Fish with Columnaris

Mouth fungus is an infection caused by a bacterium, although it looks eerily similar to a fungal infection.

It appears as white threads or splashes on the mouth or body of fish. It can be accompanied by ragged fins, rapid breathing, excess mucus production and sores and lesions in advanced stages.

Early intervention is crucial!

Treatment of mouth fungus disease:

  • Antibiotic treatment with Maracyn or Formalin;
  • Adding aquarium salt to the aquarium in one teaspoon per gallon concentration, do this every day for 3 days;
  • After symptoms subside, perform a 50-70% water change;
  • You can also create a dip solution of 10 mg/l potassium permanganate and leave your fish in it for 30 mins but be very careful not to exceed dosage and treatment time as you may end up burning your fish with this chemical.

6. Dropsy

Dropsy is a serious illness that unfortunately is untreatable. It’s an infection that causes kidney and liver problems, which result in fish retaining water.

Because of water retention, fish will look bloated with scales poking out. They will have trouble swimming. Bad water conditions and stress can cause dropsy.

Treatment of dropsy:

Treatment in unavailable for dropsy, especially because as soon as fish start to exhibit symptoms, it’s already too late for them to be saved.

Epsom salt baths have been recommended by experts and hobbyist fish-keepers, however, it will only slow down the process or make your fish feel more comfortable.

7. Swollen Gills

If you notice swollen gills on your molly fish or if your mollies coming to the surface and gasping for air, it’s a sign of bad quality water in the aquarium.

Ammonia or carbonate poisoning is known to cause swollen gills and cause fish to gasp for air.

Irregular water changes, food left to decay in the aquarium can all cause ammonia levels to spike.

Treatment for swollen gills:

  • Do a 50% water change and monitor toxin levels over the next few days;
  • Don’t feed your mollies for a couple of days until swelling subsides and to prevent further fouling of water;
  • Add some nitrifying bacteria to the aquarium (you can use API Quick Start).

8. Red Blood Spot


Molly Fish – Red Blood Spots

Typically appearing in newly cycled tanks in which the water chemistry is off. Red spots appearing on the stomach or body of your molly fish can be attributed to ammonia or nitrites poisoning.

Aquariums that are not properly cycled will often have high ammonia and nitrite levels. If you’re a beginner, familiarize yourself with the aquarium cycle, and let your aquarium age for at least 2 weeks, but preferably 6, before adding any mollies to it.

Treatment of red blood spot:

Unfortunately, the disease is untreatable and will eventually cause the death of your fish, therefore, you should do your best to prevent it by properly performing a nitrogen cycle.

If it’s in its very early stages, you may be able to save your fish, or at least some of them, by immediately taking measures (skip feeding for a day, perform water change) to eliminate toxins from the aquarium.

Continue monitoring ammonia and nitrite levels and perform regular water changes to dilute toxins that accumulate due to the waste produced by your fish.

9. Viral Haemorrhagic Septicaemia

As its name suggests, this is a viral infection that will cause lesions, ulcers, pale gills, bulging eyes, and as the disease progresses it can cause fin rotting. Infected fish will stop eating, will become darker in coloration and will eventually die.

Treatment of VHS:

  • Use antibiotics from the first signs of the disease (Maracyn 2 or API Furan 2 work well for VHS);
  • Perform a major water change after treatment.

10. Swim Bladder Disorder & Inflammation

The swim bladder is a gas-filled internal organ that helps control buoyancy of fish, helps balance fish while swimming and keeps fish at current water depth.

Although not a very common issue in mollies, swim bladder disorder can be caused by high stress and poor water conditions.

Swim bladder inflammation is a different disease from swim bladder disorder. It’s caused by a virus and it’s incurable. In fact, fish with swim bladder disorder should be immediately removed and humanely disposed of.

Fish with swim bladder inflammation have a distended belly and swim with their head pointing downwards.

11. Pop-Eye Disease


Molly Fish with Pop-Eye

Pop-eye is not a single disease, it’s more like a symptom that can be caused by various fish diseases like fungus, bacterial infections, dropsy, fish TB or internal parasites.


Pop Eye Disease – Sailfin Molly Fish

Since its root cause is difficult to determine, it’s also very difficult to treat. It’s not deadly, but it causes the eyes of your fish to protrude from their socket and even fall out.

Treatment of pop-eye:

  • If you can determine an obvious cause (e.g. bacterial infection, fungal infection, etc.) apply the treatment for the underlying cause.

12. Gill Flukes

Gill flukes is a parasitic infection caused by a tiny white worm that burrows itself into the gills of your fish causing breathing difficulty and bleeding.

Molly fish with gill flukes can be seen gasping for air at the surface of the tank or in the bottom of the tank.

Gill worms are usually introduced into the aquarium via new plants or new fish, so make sure you always quarantine new fish.

Treatment of gill flukes:

  • In its early stages, special medication to treat gill flukes is available and you should do a tank level treatment. More advanced cases, i.e., when bleeding already occurs cannot be treated.

13. Camallanus Parasite

Brown or orange in color, this parasite can be noticed sticking out from the anus of fish. New fish added to the aquarium (especially those previously kept in ponds) are the most likely culprits for proliferating the disease in your aquarium.

Treatment of Camallanus parasite:

  • The current treatment for this disease includes a 5-day treatment course with Levamisole (brand name Ergamisol) or treatment with Fenbendazole and Parcide X or D;
  • Perform a major water change (upward of 90%) after treatment and vacuum substrate too;
  • Repeat the entire process in 3 weeks.

14. Fish Tuberculosis

Caused by the bacteria mycobacterium, symptoms of fish tuberculosis include loss of appetite, ulcers around anus and on body, fading colors, rotting fins and tail.

Treatment of fish TB:

  • Fish TB is incurable and can be passed onto humans, therefore, remove sick fish and dispose of them, especially because it has a high proliferation rate and it can be even passed onto offspring;
  • Be very careful when handling fish suspected to have fish TB.

15. Bent Spine

A result of bad genetics or disease in younger fish, scoliosis or bent spine cause fish to have trouble swimming because of their crooked backs.

Molly fish with scoliosis will have trouble developing normally, may be weaker than other offspring, may be bullied by other fish and usually have a shorter lifespan.

Since it’s a result of bad genes, bent spine disease cannot be cured, it should however, be prevented by not allowing fish with this disease to breed and making sure you’re breeding healthy adults.

16. Hexamitiasis

Another parasite that can attack your mollies, Hexamitia parasite isn’t common among mollies, but there are instances when it can infect molly fish too.

Fish infected with this disease lose their color, lesions appear on head or body, they lose their appetite and have trouble swimming.

Treatment of hexamitiasis:

  • Use medicated food (metronidazole, brand name Flagyl) to treat molly fish infected with Hexamitia;
  • If your fish are refusing food, treat the water with 250 mg/10 gallons once a day for at least 3 days.

These are the most common molly fish diseases and illnesses you may encounter while keeping these fish.

You should take time to familiarize yourself with the symptoms of these diseases and treatment options, so you’ll be prepared and know what to do if your fish fall prey to one of these illnesses.

Any fish keeper will tell you that preventing diseases is 100% better than attempting to cure them, therefore, let’s see what you can do to prevent diseases, parasites, and other illnesses in your mollies.

Quick & Practical Molly Fish Disease Prevention Tips

Here is a disease prevention checklist that you should abide by to keep your molly fish strong and disease-free:

  • Make sure water temperature and other parameters are at required levels.
  • Keep the tank clean by performing regular water changes and cleaning the tank
  • Make sure the nitrogen cycle in your tank is completed and don’t rush the cycling process
  • Offer your fish a varied and well-balanced diet and avoid overfeeding at all costs
  • Remove dead fish, decaying plants, or leftover foods from the aquarium immediately
  • Monitor your fish for diseases and monitor water quality
  • Always quarantine new fish prior to adding them to the aquarium, keep them in quarantine for 3-4 weeks
  • Whenever you spot sickly looking fish, move them to a hospital tank for quarantining and treatment
  • Be careful when adding new plants to the aquarium, these can carry diseases and parasites, make sure to disinfect them first
  • Make sure you have basic medications for common molly diseases at home
  • Don’t breed fish that aren’t healthy or have genetic conditions that shouldn’t be passed down to their offspring
  • Don’t overstock your aquarium, too many fish in the tank will lower oxygen levels and cause waste and toxins to accumulate faster than you can remove them
  • Avoid stressing your fish out
  • Always check that the fish you’re housing together are compatible with each other
  • Remove fish that are aggressive or nip at the fins of other fish, injuries open your fish up to all sort of infections
  • Be careful when feeding your fish with live foods — these too can carry diseases.

While prevention does take work, it’s so much easier than stressing over diseased fish and experimenting with treatments (since many diseases can look the same, sometimes it’s not clear which treatment you should follow, see dropsy) not knowing whether it’ll work or not.

Be advised that while most fish diseases are curable, many aren’t, therefore, your only line of safety is doing all that you can to ward off potential diseases.

If you follow these measures and you’re careful with how you feed your fish and how you manage tank maintenance tasks, you’ll be able to prevent diseases in your aquarium.


I’m confident that this guide to the illnesses that can affect mollies will help you in significantly reducing the occurrence of diseases in your molly aquarium.

By taking all preventative measures I discussed and by closely monitoring your fish, you’ll be able to act fast if something were to occur.

Offer your mollies a good start in life by feeding them a healthy diet and keeping them in a well-cycled aquarium.

Clean the tank regularly, be careful when introducing new fish or decorations to the tank and avoid keeping your fish in stressful environments.

Molly Fish   Updated: March 8, 2020
avatar Hello, my name is Fabian, and I am the Chief Editor at Aquarium Nexus. I have over 20 years of experience in keeping and breeding fish. The aquarium hobby brings me immense joy, and I take great pleasure in sharing my experiences with others.

Questions and Answers

Tom Berry June 27, 2019 Reply

Thanks for all the advice, its really helpful to my 14 year old son who’s just starting on his aquarium journey …Tom.

Debbie smith November 28, 2019 Reply

One of my black mollys has gone from being really active to swimming head downwards struggling to get upright and I notice it has a white substance coming out of his anal and the anal is also white I don’t know what’s wrong and what to do for him as this has all happened in one day

    Hi! This sounds like it could be a problem with the swim bladder. If your other fish are also affected, then probably it is ammonia poisoning and you should do a 30% water change and monitor the ammonia and nitrite levels in the next hours and days. This is the most common reason, why mollies die.
    If only this fish has this problem, than you can’t do much about it, because the swim bladder is an organ which helps with the buoyancy of the fish. Swim bladder disorder can’t be treated in most cases, so you can’t do much about it.
    Make sure the water parameters are right to avoid this problem in the future.

Hi! My brother’s balloon molly has a orangy- brown pretension coming out of his side. I tried treating with aquarium salt, but it didn’t help. It almost looks like part of him is decomposing. It’s not affecting the other fish in the tank and it doesn’t seem to be affecting him very much, but his organs are close to the surface of his skin which could be bad. Any thoughts?

    This can happen due to some sort of infection. Most likely it is not treatable. If it doesn’t affect the other fish, probably it is a genetic problem with that particular molly. In any case, you should separate the sick molly from the rest.

Anahstrailya Ball February 23, 2020 Reply

Hey there, just had a Molly question. Our Molly has pop eye, is having breathing problems, is seizing up, and isn’t as active as before. We just now found what looked to be a little wriggling hair moving towards the heater. Any idea what the actual fresh hell this could possibly be??

    The cause of pop eye in fish is really hard to determine. I suggest you check water parameters first. If your tank has 0ppm ammonia and 0ppm nitrite, most likely your fish has a bacterial or fungal infection. Treating your fish with Seachem Paraguard can help treating this disease. If your other fish look healthy, than you can only treat this fish alone in a separate tank. If your other fish have sign of disease, you should treat your whole tank.

Carri Carothers October 13, 2020 Reply

My 8 yo black molly has gold blisters on his forehead. They seem to swell, then settle down to black bumps. Help plz? Thx!!!!

    This sounds like Velvet diseases. The Seachem Paraguard medication can help with this disease, but if your fish is so old, maybe no medication can help anymore.

    Sue Williams November 2, 2020 Reply

    Hi there, our gold molly has a lump protruding from its head. Its a dark colour.
    The fish is swimming and eating normally. Any ideas?

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