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Corydoras are hardy fish, but they are not immune to disease. Most health problems in corydoras are preventable and directly linked to water quality, substrate choice, or stress from inadequate school size. This guide covers the most common corydoras diseases and health issues, how to identify them, and what to do.
Barbel Erosion
Barbel erosion is the most common and most corydoras-specific health problem. Corydoras use their sensitive barbels to forage through the substrate, and sharp or coarse gravel damages them progressively over time. Poor water quality accelerates the damage by promoting bacterial infection of already-irritated tissue.
Symptoms: Shortened, stubby, or missing barbels; redness or inflammation at the base of the barbels; reduced foraging activity.
Cause: Sharp or coarse substrate; elevated ammonia or nitrite; bacterial infection of damaged tissue.
Treatment: Switch to soft sand or very fine smooth gravel immediately. Improve water quality with water changes. Mild cases may resolve on their own once the substrate is corrected. Severe cases with active infection may require antibacterial treatment. See our Corydoras Barbel Erosion guide for full details.
Ich (White Spot Disease)
Ich is caused by the parasite Ichthyophthirius multifiliis and is one of the most common diseases in freshwater aquariums. Corydoras are susceptible to ich, particularly when stressed by temperature fluctuations, poor water quality, or inadequate school size.
Symptoms: Small white spots on the body and fins resembling grains of salt; flashing (rubbing against decor or substrate); clamped fins; lethargy; appetite loss.
Cause: Parasite introduced through new fish, plants, or equipment; triggered by temperature drops or stress.
Treatment: Raise temperature gradually to 80–82°F to accelerate the parasite life cycle. Remove activated carbon from the filter. Treat with an ich-specific medication such as Ich-X, following manufacturer instructions. Continue treatment for at least 7 days after the last visible spot disappears. Perform water changes every other day and re-dose after each change.
Prevention: Quarantine all new fish for 2–4 weeks; maintain stable temperature; keep water quality high.
Red Blotch Disease
Red blotch disease is a bacterial infection that causes red, hemorrhagic patches on the body of corydoras. It is strongly associated with poor water quality and is one of the more serious bacterial conditions affecting corydoras.
Symptoms: Red or bloody patches on the body, particularly on the belly and sides; lethargy; appetite loss; the fish may rest on the bottom more than usual.
Cause: Bacterial infection, typically secondary to elevated ammonia, nitrite, or nitrate; stress from overcrowding or inadequate school size.
Treatment: Perform immediate water changes to address water quality. Treat with a broad-spectrum antibacterial medication. Isolate affected fish if possible to prevent spread and allow targeted treatment. Identify and correct the underlying water quality issue.
Prevention: Maintain ammonia and nitrite at 0 ppm; keep nitrate under 20 ppm; perform regular water changes.
Fin Rot
Fin rot is a bacterial infection of the fins and is almost always secondary to poor water quality or physical injury. Corydoras with fin rot have ragged, deteriorating fin edges that may progress to the fin base if untreated.
Symptoms: Ragged or frayed fin edges; white or red margins on the fins; progressive fin loss if untreated.
Cause: Bacterial infection secondary to poor water quality, physical injury from sharp decor, or stress.
Treatment: Improve water quality immediately with water changes. Mild cases often resolve with water quality correction alone. More advanced cases may require antibacterial treatment. Remove any sharp decor that may be causing physical damage.
Internal Parasites
Internal parasites are less visible than external diseases but can cause significant health decline in corydoras. Fish with internal parasites may appear thin despite eating, or may stop eating entirely.
Symptoms: Sunken or hollow belly despite apparent feeding; weight loss over time; white or stringy feces; lethargy.
Cause: Parasitic infection, often introduced through live food or new fish without quarantine.
Treatment: Treat with an antiparasitic medication such as Levamisole or Praziquantel, following manufacturer instructions. Quarantine affected fish if possible.
Prevention: Quarantine all new fish; avoid feeding live tubifex worms, which are a known parasite vector.
Dropsy
Dropsy is not a single disease but a symptom of organ failure, typically kidney failure, causing fluid accumulation in the body cavity. It is difficult to treat and often fatal by the time it is visibly apparent.
Symptoms: Swollen, bloated abdomen; scales raised and protruding outward (pinecone appearance); lethargy; appetite loss.
Cause: Bacterial infection, viral infection, or organ failure secondary to chronic poor water quality or stress.
Treatment: Isolate affected fish immediately. Treatment success is limited once dropsy is advanced. Epsom salt baths may provide temporary relief. Antibacterial treatment may help if bacterial infection is the underlying cause. Focus on maintaining excellent water quality for the remaining fish.
Prevention: The Most Important Section
The vast majority of corydoras health problems are preventable:
- Use soft sand or very fine smooth gravel — prevents barbel erosion, the most common corydoras-specific problem
- Maintain water quality — ammonia and nitrite at 0 ppm, nitrate under 20 ppm, weekly water changes
- Keep a school of at least 6 of the same species — reduces chronic stress
- Quarantine all new fish for 2–4 weeks before adding them to the main tank
- Avoid live tubifex worms — use frozen instead
- Cover filter intakes with a pre-filter sponge to prevent injury