Five fish diseases account for roughly 90 percent of aquarium illness cases. An aquarist who can identify the disease within 24 hours of symptom onset and apply the correct medication saves most fish. An aquarist who waits a week and applies the wrong treatment loses most fish. This guide covers ich, fin rot, columnaris, velvet, and dropsy with symptom photos descriptions, the specific medications that work, and the treatment schedules to follow. The same five diseases appear in both freshwater and saltwater tanks (with species-specific organisms) and use overlapping treatments.
Disease 1: Ich (white spot disease)
Pathogen. Ichthyophthirius multifiliis in freshwater, Cryptocaryon irritans in saltwater. Both are parasitic protozoans.
Symptoms. White salt-grain sized spots on body, fins, and gills. Fish flashing (scratching) against rocks and substrate. Rapid gill movement. Reduced appetite in moderate to advanced cases. Spots appear over 2 to 4 days and multiply rapidly.
Lifecycle (this matters for treatment). Adult parasite drops off fish, encysts on substrate or filter, releases 100 to 1000 tomites that swim and infect new hosts. Treatment must hit the free swimming stage because adult cysts and on-fish parasites are protected.
Freshwater treatment.
- Method 1: heat and salt. Raise temperature to 86 degrees over 24 hours. Add 1 tablespoon aquarium salt per 5 gallons. Hold 14 days past last visible spot. Daily 25 percent water changes during treatment.
- Method 2: medication. API Super Ick Cure, Kordon Ich-X, Hikari Ich-X. Dose per package, treat 14 days minimum, water changes between doses.
Saltwater treatment.
- Cupramine in QT at 0.5 ppm for 21 days minimum.
- Tank Transfer Method: move fish to a clean tank every 72 hours for 12 days (4 transfers). Parasites cannot complete lifecycle.
- Hyposalinity: lower salinity to 1.009 SG for 21 days. Effective but stresses fish.
Disease 2: Fin rot
Pathogen. Bacterial (typically Aeromonas or Pseudomonas). Often opportunistic, secondary to water quality issues.
Symptoms. Frayed, ragged fin edges. Black or white discoloration along fin margins. Fins progressively recede from outer edge toward body. No spots, no fuzz, no body lesions in early cases.
Underlying cause. Almost always poor water quality. Check ammonia, nitrite, nitrate before treating. If nitrate is over 40 ppm or ammonia/nitrite are detectable, water quality is the cause and medication alone will not work.
Treatment.
- Step 1: Improve water quality. Water change 50 percent, then 25 percent every 3 days for 2 weeks.
- Step 2: Medication. Seachem Kanaplex or API Furan-2 in hospital tank. Treat for 5 to 7 days per package.
- Step 3: Aquarium salt (freshwater) at 1 tablespoon per 5 gallons during recovery.
- Recovery: fin regrowth takes 4 to 8 weeks. New tissue grows clear and gradually colors in.
Disease 3: Columnaris (cotton wool disease)
Pathogen. Flavobacterium columnare. Highly contagious bacterial infection.
Symptoms. White or grayish cotton-like patches on body, fins, mouth, or gills. Patches may be raised. Fish often refuse food and become lethargic. Frayed fins similar to fin rot but with the characteristic fuzzy patches. Mouth columnaris (cottonmouth) is particularly dangerous.
Lethality. Acute columnaris kills fish in 24 to 72 hours. Chronic columnaris may take 1 to 2 weeks. Highly contagious in a community tank, especially at temperatures above 78 degrees.
Treatment.
- Lower temperature to 75 to 76 degrees to slow bacterial reproduction.
- Move affected fish to hospital tank immediately.
- Kanaplex (Seachem) for 5 days, or API Furan-2 plus API Erythromycin combination.
- Aquarium salt at 1 tablespoon per 5 gallons.
- Treat the display tank prophylactically with kanaplex because columnaris spreads to tank mates.
Disease 4: Marine velvet (Amyloodiniosis)
Pathogen. Amyloodinium ocellatum. The most lethal marine parasite. Saltwater only.
Symptoms. Fine gold or rust colored dust on fish (much finer than ich spots). Heavy rapid breathing as gill function fails. Fish hide in corners. Death within 24 to 72 hours of advanced symptoms in untreated cases.
Identification. Shine a flashlight at the fish from the side. Velvet appears as fine glitter that ich does not produce. Ich spots are larger and more discrete.
Lethality. 80 to 100 percent in 5 to 10 days untreated. The worst aquarium parasite for stocked reef tanks.
Treatment.
- Cupramine at 0.5 ppm in QT for 30 days.
- Chloroquine phosphate at 60 mg per gallon if available (prescription only in some regions).
- Tank Transfer Method works but velvet has a faster lifecycle than ich (3 to 4 days), so transfers happen every 48 hours.
- Display tank goes fallow for 76 days minimum to starve out remaining parasites.
Disease 5: Dropsy (edema)
Pathogen. Generally bacterial (Aeromonas) plus internal organ failure. Sometimes viral.
Symptoms. Swollen body with raised scales (pineconing appearance). Bulging eyes (pop eye). Loss of appetite. Lethargy. Sitting on bottom.
Reality. Dropsy is often the final symptom of internal organ failure. Treatment success rate is 20 to 40 percent even with aggressive intervention. Catching early matters more here than for any other disease.
Treatment.
- Move to hospital tank with epsom salt at 1 tablespoon per 5 gallons (assists osmotic balance).
- API Furan-2 plus kanaplex combination for 7 days.
- Medicated food (Seachem Focus plus Metroplex) if fish still eats.
- Improve water quality and feed high quality food.
If fish are still alive after 14 days of treatment, recovery is possible. After 21 days without improvement, euthanasia is the humane choice (clove oil overdose).
Prevention beats treatment
Most disease outbreaks trace to two causes: new fish without quarantine, and chronic water quality issues. Prevention checklist:
- Quarantine every new fish for 30 days (see /articles/quarantine-tank-setup/)
- Test ammonia, nitrite, nitrate weekly
- Water change 20 to 30 percent weekly
- Feed varied high quality food (frozen plus pellets, not flakes only)
- Stock conservatively, avoid aggressive species mixing
- Maintain stable temperature (avoid swings over 2 degrees daily)
- Keep a hospital tank ready to deploy (sponge filter cycled in main tank sump)
A tank that follows this checklist sees 80 percent fewer disease cases than a tank that skips QT and runs high stock densities.
When to euthanize
Some cases do not respond to treatment. Humane euthanasia uses clove oil:
- Mix 1 ml clove oil with 1 cup tank water
- Add to a 2 cup container with the fish
- Fish loses consciousness in 1 to 2 minutes
- Add 4 to 5 times the original dose to ensure full euthanasia
- The fish does not suffer when this protocol is followed correctly
See our aquarium water parameters explained for the water testing that prevents most disease, and quarantine tank setup for the QT protocol. The /methodology page documents our testing protocols.
Frequently asked questions
What is the most common aquarium fish disease?+
Ich (Ichthyophthirius multifiliis in freshwater, Cryptocaryon irritans in saltwater) accounts for roughly half of all aquarium fish disease cases. It presents as white salt-grain spots on body and fins. The good news is ich is treatable with a 14 to 28 day protocol. The bad news is ich is the disease most often misdiagnosed early when fish are still saveable.
How long does fin rot take to heal?+
Fin rot caused by bacteria responds to medication (kanaplex, furan-2) within 5 to 7 days. Fin regrowth takes 4 to 8 weeks for the tissue to fully regenerate. The shredded fin edges turn black or white at first, then begin clear regrowth. If fins continue to recede during treatment, the underlying water quality issue has not been fixed.
Can fish disease wipe out a whole tank?+
Yes. Marine velvet (Amyloodinium) wipes 80 to 100 percent of fish in a saltwater tank in 5 to 10 days if untreated. Columnaris in a freshwater tank can wipe 50 to 70 percent in 7 to 14 days. The wipe rate depends on tank density, water temperature, and immediate response. Early identification matters more than the specific treatment.
Should I medicate the display tank or move sick fish to a hospital tank?+
Move them to a hospital tank when possible. Medications damage biological filtration, kill invertebrates and corals, and persist in substrate. A 10 to 20 gallon hospital tank with bare bottom, sponge filter, and heater costs 80 dollars and protects the display. The exception is whole-tank parasitic outbreaks where moving fish only delays the inevitable.
Is salt safe to use as a treatment for freshwater fish?+
Aquarium salt (sodium chloride, no additives) at 1 tablespoon per 5 gallons treats ich, mild fin rot, and aids osmotic balance during stress. It is safe for most tropical community fish. Salt is NOT safe for scaleless fish (loaches, plecos, corydoras at high doses), most live plants, and most freshwater invertebrates. Use salt in a hospital tank when livestock allows.
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Riley Cooper
Riley Cooper writes for The Tested Hub.