02-25-2014, 08:20 PM
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I just wanted to add - the new fish not being the first to die doesn't mean that they didn't bring something with them. It's actually more likely that if they were carrying something that they had been living with for a while, and the fish that were in the tank already had never been exposed to this pathogen, that they would not be the first fish to show symptoms. You could say that they are carriers. Once other fish (who have never encountered the parasite or whatever was causing the illness, and thus have no defenses against it) start dying, the added stress could push them over the edge and they also succumb. What you described is pretty suggestive of some kind of infectious disease process.
Frankly, I would think that it's easier to bleach the tank, get rid of anything that can't be bleached, and start over. I don't like to take chances with anything infectious, and you do not have any fish that need to be housed while you're attempting to clean.