Bactrial infection develops in an established system...why?

[Rare Angels]

Hello ,



I have a question concerning fish developing bacterial infections in an established system. The fish were all moved from individual tanks to a central breeding system. After 2 months several of them have developed a bacterial infection.



I have treated new fish for bacterial infection while in QT but I have never had a bacterial infection arise in established fish and system. The new breeding system had been running for several months with roughly the same bio-load.



Do I just treat the sick fish or do I treat the entire sytem? This is something I haven't run across before.



Thanks for the help



Dave