I would also bet, you didn't quarantine the new fish, and one or a few may have been disease carriers which is spreading throughout the tank.
Could be anything from a bacterial, fungal or viral infection, .
If bacterial if your temp is 82'F or above, this could be making it more virulent.
If its viral, no medication is effective.
And if bacterial you may need something antibiotic, but antibiotics are usually specific to certain types of bacteria, you would need to know if the bacteria is gram - or gram + before choosing a medication.
Where did the new fish come from?