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Why Did My Fish Die?

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A question usually posed by beginners, even experienced saltwater aquarium hobbyists are sometimes stumped as to the reason a fish in their tank has died.

Normally, an alert aquarist can see the symptoms (frayed fins, cloudy eyes, unusual behavior, water test results) of a problem before a death occurs, but ocassionally a fish will die suddenly with no apparent reason. Barring the possibility of a fish becoming infected and dying from the West Nile Virus, below are the most probable reasons for a tank critter to die.

  • Ammonia Poisoning
    • Occurs most frequently while cycling a new tank, but also happens during collection or while in transit.
    • If the tank has been fully cycled for a period of time, New Tank Syndrome may be the cause.
    • Symptoms include cloudy eyes, frayed fins, rapid gilling and lack of appetite.
  • Cyanide Poisoning
    • Almost exclusively occurs in fish captured in certain areas of the South Pacific.
    • May take a week or two to show up after collection.
    • Other than a lack of appetite, there are few symptoms to detect.
  • Poisonous Sting
    • May be inflicted from a poisonous fish(i.e. Lionfish) or from a poisonous anemone.
    • Difficult to diagnose.
    • Symptoms include erratic behavior and lack of appetite.
  • Other Poisons
    • Anything from cigarette ashes to hair spray can be sources.
  • Malnutrition
    • Most often occurs when a new fish is introduced into a tank.
    • Lack of appetite due to disease.
    • First symptom is a sunken belly.
  • Oxygen Deprivation
    • Normally caused by lack of vertical water movement in a tank.
    • Initial sign is fish staying at the surface of the tank water.
  • Old Age
    • Inevitable.
    • Difficult to diagnose.
  • Physical Injuries
    • Pretty obvious.
    • Usually caused by other tank occupants.
  • Bacterial Infection
    • Symptoms include cloudy eyes, red areas on the body, swollen belly (internal infection).
  • Bends (Diver's Disease)
    • Rare for the hobbyist to see.
    • Normally occurs during collection.
  • Other Diseases
    • Ick, fungal infection.

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