fish such as feeder goldfish have been exposed to a lot of bacterial strains such as aeromonas and have built up an immunity/resistance to the disease and although they may not show any symptoms can be carriers. the tilapia have possibly not been exposed to the disease and will succumb to it.

the hatcheries use a lot of antibiotics to keep the fish alive and the strains of bacteria that the fish carry are many times are resistent to different antibiotics.

there are aeromonas strains that can wipe out a hatchery in a very few days. anyone that remembers the angelfish/discus plague will know what i am talking about.

i believe that if you bought the goldfish at a top quality KOI dealer that you would not have that problem.