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What's Wrong with My Fish? A Look at Aquarium Diseases

If one or more of your fish begins acting strangely or has changes in its appearance, it may have developed a disease. When you have a sick fish, there are certain steps that should be taken, no matter what the illness is.

  • Quarantine the sick fish so the disease doesn't spread to other fish.
  • Do a 20-25% water change in the main aquarium.
  • Test the water for ammonia and nitrites. Most diseases are a sign of poor water conditions.
  • Raise the temperature of both the main aquarium and the quarantine tank two or three degrees to help kill whatever is causing the illness.
  • Add a bit of salt. Freshwater fish can handle small amounts of salt and it can be very helpful in killing and preventing disease. Use one teaspoon of marine salt per 10 gallons of water.

While the following information is not a substitute for proper diagnosis by a vet or aquarium expert who can have a look at your sick fish, this list can help you understand what you may be seeing in your tank.

Disease: Aeromonas or Hole in the Side
Symptoms: Red spots on the side, growing into open sores. Also visible red streaks on the fins or side, loss of scales and flesh.
What it is: Infection, leading to septicemia and death. Bacterial versions can be cured, viral versions cannot.
Common treatment: Dual treatment with medications for both infection and fungus, to prevent additional infection in the sores.

Disease: Branchiomycosis
Symptoms: Gills become spotted with brown patches and tear off. Fish gasps, trying to breathe.
What it is: Fungus
Common treatment: Medications available. Do not return fish to main aquarium until its gills have regenerated and repaired in the quarantine tank.

Disease: Chilodonella or Slime
Symptoms: Slimy spots or film on scales.
What it is: Parasite, commonly affects fish such as Angels and Discus but can be seen in others.
Common treatment: Medications available, usually a 10-day course of treatment is recommended.

Disease: Columnaris
Symptoms: Fins fray and shred. Cloudy white mold on scales, similar to fungus but treatment won't work. Gills may turn brown. Often appears as white worms on the aquarium walls before fish show signs of illness.
What it is: Parasite, usually with secondary bacterial infection
Common treatment: Medications for parasite plus antibiotic for infection.

Disease: Corneybacteriosis
Symptoms: Eyes bulge outward.
What it is: Bacterial infection resulting in swelling inside the head, making the eyes bulge.
Common treatment: Antibiotics

Disease: Dropsy
Symptoms: Bloated body, scales often standing out from body.
What it is: Caused by an internal bacterial infection.
Common treatment: Antibiotic (usually tetracycline)

Disease: Fin or Tail Rot
Symptoms: Long fins become shredded and look eaten away.
What it is: Bacterial infection
Common treatment: Antibiotic (usually tetracycline)

Disease: Flukes
Symptoms: Behavioral—fish will scratch itself against aquarium walls and furnishings, attempting to remove the problem.
What it is: Parasite.
Common treatment: Medications available. Aquarium salt baths can also kill flukes.

Disease: Fungus or Cotton Wool (called Cataracts on the eyes)
Symptoms: White or gray fuzz resembling cotton or wool.
What it is: Fungal infection, usually appearing on injuries to fins or scrapes. Can also appear on the eyes and mouth.
Common treatment: Easy to treat with many medications. Infection may also be a sign of an aggressive fish picking on the one that is ill, so watch for nipping and possibly remove the bully.

Disease: Hexamita or Hole in the Head
Symptoms: Small cyst resembling a pimple appears on the head, grows into a big open sore that is deadly if not treated.
What it is: Parasite which can affect any fish but commonly infects cichlids.
Common treatment: Treat with medications for this disease and may also want to treat for fungus to prevent additional infection.

Disease: Ich or White Spot (Ichthyophthiriasis)
Symptoms: Small white spots that resemble salt or sand. Usually appears on fins first.
What it is: Spots are growing parasites that quickly spread to other fish in the tank.
Common treatment: Medications available, but must be used carefully because they can harm the fish as well as treat the disease. Must wait until parasites have broken out of the sacs (spots) on the fish, but treat immediately once they do. Will often have a second outbreak days later requiring additional treatment.

Disease: Lernea or Anchor Worm
Symptoms: White cysts resembling pimples with longer bits of white hanging out of the cysts.
What it is: Worms
Common treatment: Medication, several available.

Disease: Oodinium or Velvet
Symptoms: Fine white powdery spots. Looks similar to Ich, but appears on the body more than the fins.
What it is: Spots are growing parasites that can spread to other fish in the tank.
Common treatment: Medications available. Must wait until parasites have broken out of the sacs (spots on the fish). Life cycle of Oodinium is slower than Ich and easier to treat.

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