12-10-2009, 12:45 PM
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This is one of those topics on which there are several schools of thought. There are several aspects worth noting, then one has to decide for themselves how long to quarantine, if at all.
First issue is to understand why you are quarantining. This may sound silly, but the fact is that 4 weeks is not going to bring everything to light. I have read serious aquarists who QT for 3-4 months. Even this may not be adequate, from my own experience. So, decide what you are hoping to achieve and QT accordingly. If it is a parasitic issue like ich, velvet, or similar, 3-4 weeks should be sufficient.
As for ich, I have known it appear several weeks later out of nowhere, or so it seems. I believe it is actually in the tank all this time, and the fish naturally defend themselves and fight it off so we never notice it. I was reading an article by a biologist recently who mentioned that some fish seem to develop an immunity after an initial exposure that they have fought off. We all know that some fish are more prone to ich than others that never get it. And the general consensus is that ich is always caused by stress; fish that are not under stress can usually repel it.
In your case Angel, I believe as I mentioned in that other thread that the cardinals came down with it out of stress at being placed in a new tank that was not matured sufficiently for such sensitive fish. I've had the same thing occur. But never in an established tank. And there are species of fish that always die off if introduced to biologically immature aquaria--pygmy corys, many of the pencilfish, and some tetras.