For example, pigs harbor high amounts of Escherichia coli bacteria in their digestive tracts (we do too).  This bacteria doesn't bother the pigs but will infect other animals exposed to their waste - remember over the past year when there was an E. coli outbreak in spinach?  This outbreak was caused by humans ingesting spinach plants contaminated with bacteria-laden waste from a pig farm.  This is a prime example of inter-species cross-contamination.

Fish aren't pigs (though I'm sure some of us who spend too much money on fish food could argue ;) ).  But the premise is the same; just using that for an easy example. 

Let's translate this into fishspeak.  Say that a goldfish has a species of bacteria living on its skin, within its slime coat.  This bacteria feeds off organic particles in the slime coat, but goldfish are adapted to this bacteria and aren't harmed by it, just like we harbor bacteria in our bodies that don't hurt us.

Now say that you put a betta in a goldfish tank.  This is where the problems will start.  For one, the temperature can't be right for both fish.  It'll either be too high for the goldfish or too low for the betta.  In our example, the betta's in the goldfish tank so the temperature is at least six degrees too low for the betta.  Bam.  Stressed betta fish.

Suddenly, the bacteria on the skin of the goldfish (that are also in the water, just from contact) have a whole new fish to play with.  They're thrilled and begin to multiply like crazy as they take in nutrients from their new host.  But the betta isn't nearly as thrilled.   He is not used to the bacteria that live on goldfish,  because in the wild he would never even meet a goldfish.  And suddenly the bacteria that are benign on your ryukins or comets are virulent on your betta.

See how this works?  The cross-contamination can happen over a variety of media - skin, digestive wastes, etc, and can occur in any species of fish not found in each other's proximity in the wild.  It's just more common with goldfish vs tropicals because their differing temperature requirements make it certain that at least one of the two species will be stressed already due to temperature, and thus more susceptible to disease.

If life gives you lemons, squeeze them into people's eyes and run away!

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It is observable that when an illness, say ich, strikes a tank there can often be some species of fishes that are not at all affected. I have always just chalked it up to different fish having different immune systems, and to having been "challenged" before and having some immunity because of that -- the old "what doesn't kill you makes you stronger". I have also learned, through years of handling fish on the wholesale level, that it is very dangerous to add a new batch of fish to a tank that already has fish in it. This is why most wholesalers always introduce new fish into a tank that does not have fish in it.

Your explanation is very interesting -- and it definitely makes sense.

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Be warned -- everyone at college has a weird roommate. If you don't have a weird roomate -- then you're the weird roommate.
                                                                                                  Conan O'Brien, Stuyvesant High School.

The reason it's inadvisable to add a wholesale shipment of fish to an already-established batch is immunology 101.  Put very simply, the first time we, or a fish, are exposed to a pathogen, our immune system takes too long to produce antibodies to the pathogen and we get sick.  From that point on, however, we will possess fast-acting antibodies that react quickly to an invasion by that same pathogen.  Thus, when we come down with the chicken pox as children (well I did, hooray for modern vaccines), it's rare for us to get the disease again.  And as a result, you described it well when you stated that "what doesn't kill you makes you stronger."

So adding a new wholesale fish shipment to an established group would expose both stocks of fish to illnesses that neither one have experience immunity to.  If there were just a few fish put together - like in a home aquarium setting - the fish's immune systems have time to cope; they face the invasion with little to no side effects and their systems are primed with memory cells ready for the next time they meet that germ.  But adding so many fish together, like in a pet store, would result in all the fish being exposed to massive amounts of pathogen, which they might not be able to fight off.

I don't think that I have ever seen this type of immunology explained in an aquaculture reference book.  This is basically applied science, and of a somewhat diminished technical nature.  I could explain it more thoroughly, but at the same time there will be differences in what occurs in other biological systems versus those of fish.

A component of my knowledge comes from extensive research of agricultural techniques - that's why I vaguely mentioned farming in the previous post.  Agriculture relies on many of the same principles as aquaculture when it comes to resource management and also disease transmission.

In addition, I'm very well-studied in human and animal disease transmission and immunology, short of pursuing a degree in the field.  In my case this knowledge comes from necessity; my boyfriend has a rare genetic disease that renders him extremely immunocompromised.  Ignorance on the subject could put his life in serious danger, so I have made every effort to be well-informed.