That introduces another aspect of stress that I forgot previously. The new fish may have brought the parasite (ich), but if the fish in the tank had been healthy, they would have managed to fight it off. Being weakened by the other issues, they can't. It all goes back to keeping fish in the proper environment.
Byron Hosking, BMus, MA
Vancouver, BC, Canada
The aquarist is one who must learn the ways of the biologist, the chemist, and the veterinarian. [unknown source]
Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]