I never use any med unless I know the disease, and I'm sure it is effective against that particular disease..
1 week is not quarantine to me, and it looks like you may have deduced this as well, for your recent deaths.
My normal quarantine is 2-4 months, 2 months with an absolutely perfect appearance, anything suspicious at all, 4 months.
A suspicious clue might be a sore, an attempt to scratch, or simply being listless.
Many diseases can take months to show symptoms.
A friend who runs an aquatic exhibit at a zoo, quarantines minimum 6 months.
For me a bacterial symptom like a white sore, or listlessness for something such as columnaris will cause me to euthanize. Something simply treated like ick, or a disease that requires no meds at all like lymphocystis, or a non-lethal wound, I would attempt to save.
The colomnarus bacteria and others like it can survive dormant in dried mud on tank glass for months, and easily reinfect the system when water is added. so I would euthanize the fish, and bleach anything it came into contact with (tank, nets, tubes, everything).