Molly fish are generally hardy, but like all aquarium fish they can fall ill when water conditions or care slip. The table below covers the most common diseases, their symptoms, and treatments. Keeping water quality high is the single best preventive measure for nearly all of them.
| Disease | Symptoms | Treatment |
| White Spot Disease/Ich | White spots on body, Clamped fins, Scratching against surfaces, Lethargy and loss of appetite | Increase water temperature, Salt treatment, Medicated baths, Anti-Ich medication, Quarantine infected fish |
| Fin & Tail Rot | Ragged or punctured fins, Change in color, Incremental degeneration | Anti-bacterial or anti-fungal treatments, Improve water condition, Vitamin supplements, Quarantine infected fish |
| Swim Bladder Disorder | Difficulty swimming, Abnormal behavior, Less active | Change diet, Lower water depth, Medication, Fasting |
| Velvet (Oodinium) | Velvety coat, Scratching behavior, Clamped fins and lethargy | Anti-parasitic medication, Increased temperature, Darken the environment, Quarantine infected fish |
| Columnaris | Cotton-like patches, Altered breathing and diminished activity, Reduced food intake | Use of antibacterial medication, Improving water quality, Raising the temperature, Salt treatment |
| Gill Flukes | Rapid gill movement, Scratching behavior, Red or swollen gills | Anti-parasitic medication, Salt bath, Increase water temperature, Quarantine infected fish |
| Dropsy | Bloating or swelling, Pinecone-like scales, Lethargy and loss of appetite | Antibacterial medication, Epsom salt baths, Improve water quality, Isolate infected fish |
| Black Spot Disease | Appearance of black spots, Scratching behavior, Reduced appetite and lethargy | Anti-parasitic medication, Quarantine infected fish, Tank cleaning and water change |
| Constipation | Bloating, Less or no fecal matter, Swimming difficulty | Diet change, Fasting, Epsom salt bath |
| Scoliosis (Bent Spine) | Bent or curved spine, Swimming difficulty, General weakness | Improved diet, Antibiotic treatment, There's no guaranteed cure |
| Fish Tuberculosis | Weight loss and lethargy, Loss of scales and skin color, Lesions and ulcers | Antibiotics, Isolate infected fish, Euthanasia in severe cases |
| Popped Eyes | Protruding eyes, Cloudy eyes, Swimming difficulty or lethargy | Water change, Antibiotic treatment, Isolate infected fish |
| Camallanus Internal Worm | Visible worms, Weight loss and decreased appetite, Lethargy | Anti-parasitic medication, Water changes and tank cleaning, Quarantine infected fish |
| Viral Hemorrhagic Septicemia (VHS) | Hemorrhaging, Decreased activity and appetite, Protruding eyes or bloated belly | No established cure, Isolate infected mollies, Supportive measures |
| Swollen Gills | Enlargement or redness of gills, Accelerated gill movement, Diminished appetite or lack of energy | Change of water and evaluation of quality, Application of antibiotics or anti-parasite treatment, Segregation of infected fish |
A Note on Prevention
Most of the diseases above are triggered or worsened by poor water quality and stress. Consistent water changes, stable temperature (72–82°F), and a low-stress environment go a long way. Many hobbyists also add a small amount of aquarium salt to Molly tanks, as Mollies tolerate — and often thrive in — slightly brackish conditions that inhibit many common pathogens.
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