Did you quarantine the new fish before adding them to the tank?
Were they new to your tanks, or from another tank in your house?
It looks like a bacterial infection to me.
I'm going to assume you didn't quarantine, unless you tell me different, and say this...........
if you didn't quarantine, or the quarantine was too short, then the new fish could have introduced the disease.
I usually quarantine at least 3 months, many bacterial diseases don't present symptoms in shorter periods of time.
Another thought, you stated you had this before......maybe your surviving fish built up immunity over time, immunity that the new introductions don't have.
Quarantine is as much about what may have previously been in your tank, as much as it is about keeping new diseases out.
And if an infection becomes strong enough, it can also overwhelm the immunity, your old fish have built up.