I'm definitely not the person to provide advice on fish diseases.
However, I suggest not to overestimate the counter-argument that no new fishes were introduced. Many parasites are commonly present and don't stand a chance against immunity system. And then, one day, something happens, the host gets stressed, and the parasite gets its opportunity.
But again, I'm not good at fish diseases. I only wanted to turn your attention to that possibility. If I knew what's your problem, I'd tell you straight away. But I don't.
(Generally, I'm rather skeptical toward applying potent medications, namely when a small number of common fish is under attack. You can call me cynic but if you take into consideration the price of medication, the damage it may cause to the whole tank, full of expensive plants etc. ... Sometimes it's better to endure some losses. Just my point of view, of course.)