It’s inaccurate to say “Blue Dempseys die from inbreeding”… But it is completely true to say “Blue Dempseys are genetically weaker due to excessive inbreeding”…
If you look up some of the common results of excessive inbreeding you will see a list of problems common to Blue Dempseys…
http://en.wikipedia.org/wiki/Inbreeding#Results_of_inbreeding
The main result of inbreeding, or trait common to Blue Dempseys, we are discussing here is their lowered immune system… primarily their resistance to parasitic infection…
Using a UV light is by no means “necessary”, but is your best insurance at keeping parasites out of your system. As to where they come from… they are everywhere. As described, they thrive in areas most of us would consider “dirty” and very much so in a dirty fish tank. The cleaner the fish tank is the fewer parasites there will be in the tank to attack the Blue Dempsey thus the higher the chance of it’s immune system being able to combat the attack.
Also, as mentioned, not introducing feeder fish, or any new fish for that matter, will decrease the likelihood of transferring parasites in from another tank/system.
DirtyBlackSocks, I’m not suggesting you are “wrong” by what you said, only that in my opinion you oversimplified a true fact to a point it could easily be misunderstood…