Fish Diseases

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Every aquarist will, at some point, have to deal with a sick fish. This entry will discuss how to do it properly, and, more important - how to avoid it.


Pathogens will be almost always present in your tank, but a fish with a healthy immune system will have no problems handling them. Too much stress to the fish's health can cause a disease.

Common causes for disease that should be avoided are:

  • Bad water quality: too high values of ammonia, nitrites, or nitrates
  • Not enough oxygen
  • Too aggressive tank neighbors
  • A packed tank (too many fish)
  • Too high, too low, or fluctuating pH or temperature
  • Improper nutrition (overfeeding, wrong or unvaried food)


If you can afford it, it is a good practice to put any new fish in a separate quarantine tank and make sure they don't develop any diseases before introducing them to their new mates in the main tank. You can do fine without quarantine, but if you're keeping expensive fish, it's worth the cost of setup.

Another benefit of having a separate, uninhabited but running tank, is its possible use as a "hospital" tank. Many fish need to be alone to fight diseases. Many kinds of medicines need to be put in the water and it's better not to do that in the main tank, as they might stress the other, healthy fish.


To be able to know when a fish is sick, you have to know its normal behaviour and looks. What is your fish's feeding pattern? Is it usually active or still? Any deviations from the normal may be due to a disease.

If a fish doesn't move for a long time, refuses to eat for more than two days, swims in an abnormal way, moves with fins held too close to the body, stays at the surface gasping for air, or has spots and other unusual objects on its body, then it probably has some kind of a disease. Here is a list of common diseases and the way to cure them:

Common fish diseases and other problems

Poor water quality

If many or all fish in the tank are very inactive and gasp at the surface for air, the reason may be a high level of toxic substances or too little oxygen dissolved in the water.


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