Whenever a fish is netted, bagged and shipped (or driven home) it is a highly stressful adventure for him. He loses his immunity to disease for about 72 hours after the trip. He is weaker than another fish that has not suffered through the same ordeal. During this stressful 72-hour period his general health and body system is at a critical low point.
If, during this 72-hour period, he is exposed to parasites, other fish with health problems, unhealthy water quality and/or he has an underlying health problem of his own his chances of overcoming any of these obstacles are slim – much slimmer than that of another fish who has been “at home” in the pond for many weeks or more. Because of this it is wrong to assume “the new fish” is the culprit of any gill disease or parasitic conditions even if ALL the new fish died and the others are still alive.
Here’s a fine example:
Recently a customer purchased a number of new fish from us. Within a week every one of the new fish had died or were suffering. Naturally our first inclination was to say that something was wrong with the new fish. Upon examining a live fish from the customer’s pond we found gill flukes – a particularly nasty parasite that can only be detected through a microscope. Again – the natural inclination was to blame the new fish since they all died or were sick. Surely the new fish must have had gill flukes when the customer took it home.
We are particularly careful about quarantining and treating all our stock before they go out our door and it literally shocked me to think that we could have sold a customer a fish with gill flukes! In cases like this the first thing we have a mucus scraping performed on a fish from the same batch. It did not surprise me to find no gill flukes at all.
Then why was my customer losing only the new fish and the others seemed fine?
The others were, indeed, in trouble though it appeared at the time nothing was wrong with them. These new fish were like the canaries the miners used to carry down into the mines with them. If the air quality was bad the canary died and the miners knew they could not stay inside the shaft. These new fish were frail and died first. In this case our customer did the right thing by reacting quickly by treating his pond and killing the flukes before they could wear down the stronger fish.