Buying “Sick Fish”

for the week of 10/13/99

Many hobbyists are under the mistaken impression that it is possible to have fish that are absolutely free of parasites, bacteria and other possible disease-causing organisms. However, that is no more possible in the aquatic world than it is in our human one. Every fish has a few parasites, and plenty of bacteria inside and out: every fish in your tank, every fish in our tanks, and every fish in nature.

Does that mean that every fish in the world is sick? Of course not. A few parasites cause virtually no harm; in fact, it’s not in the best interest of a parasite to cause severe harm to its host. On the other hand, when a fish is stressed (by sudden changes in water temperature or chemistry, for example), it’s immune system is impaired, and the handful of disease organisms are able to reproduce in great numbers.

This means two important things to the aquarist. First, obviously “sick” fish should never be added to an existing tank, where they can become disease factories, producing so many organisms that can overwhelm the previously healthy existing fish. But it is also important to realize that even healthy new fish may bring new pathogens into a tank – and that new fish will also be exposed to new parasites from the existing fish, even if they, too, are healthy.

This exchanging of “germs” can in itself lead to a disease outbreak, but rarely a very serious one. But if additional stress (such as aggression, new fish being chilled on the way home, or fish being exposed to dramatic changes in water chemistry) is encountered, an epidemic may ensue, affecting both old and new specimens.

Aquariums should be inspected closely for signs of infection for a few days after new fish have been added, and the hobbyist should be prepared to act promptly if such signs should appear.

“Tip of the week” appeared regularly in 1999 and 2000.