Aquatic Health E-mail

Common Diseases In Your Fish Tank

Here's a very quick summary of symptoms and information about the most common diseases that can affect our gilled friends – there's lots more information available online and in books: Here's a very quick summary of symptoms and information about the most common diseases that can affect our gilled friends – there's lots more information available online and in books:





  • Red or bleeding gills

A sign of incurable ammonia poisoning, usually caused by a new tank that doesn't have a nitrogen cycle going yet, or an old one that's lost its beneficial bacteria. There's not much you can do for these fish, unfortunately; educate yourself about biological filtration and try again.

  • Spots or streaks on the body which appear red or orange
  • Fungus-like growths, or a white or gray film on the body.
  • Dropsy (bloating)

Evidence of various bacterial disorders. Salt baths, or antibiotics such as tetracycline, penicillin and naladixic acid can be used to treat affected fish in a separate hospital tank. Remember that the antibiotics will kill your good bacteria too – don't add them to your main tank water!

  • Tiny black specks on the body

A sign of "Black Spot" or diplopstomiasis, an infestation of parasite larva. Medications are readily available at any fish store.

  • White or gray film on the eyes

Cataracts happen when fungus grows on the eyes – it happens most frequently when your water is high in ammonia or nitrates. Correct water chemistry and treat with an aquarium fungicide.

  • Bulging "pop" eyes

Corneybacteriosis usually happens when poor water quality is caused by overcrowding. Remove some fish to a separate tank, correct water chemistry, and treat sick fish with antibiotics like penicillin and tetracycline.

  • Cottony white substance appearing on surface injuries or mouth

Common fungal infection; exacerbated by poor water conditions and/or "tank bullies" which cause injury to other fish. Correct water chemistry and treat with an aquarium fungicide.

  • Small white granules or pimples, mostly on the fins

This is the dreaded "Ich", or Ichthyophthiriasis, an infestation of parasites. It's difficult to treat because the protozoans can only be destroyed after the pimples break open, and they often mature enough to infect other fish before tank treatment is complete. There are a handful of medications and preventive treatments available in stores; read instructions, use quarantine carefully, and ask for advice from fish store employees and fellow hobbyists about your particular tank situation.

  • Black patches on the body and fins, with or without body swelling

This is a myxobacteriosis infection, uncommon but easily medicated. Commonly caused by overcrowding and poor water quality.

  • Powdery white speckles on body skin

Velvet, or oodinium, is a parasitic disease similar to Ich, but easier to treat with commercial medications. Often caused by chilling or transportation stress.

  • Visible worms, flukes or lice on the body

Simple parasitic infestation. Remove from tank, physically remove visible parasites, and treat with commercial medications. Follow instructions carefully; lack of follow-up treatment may cause re-infestation or fungal or bacterial infections.