red sore = dead fish

[PatrickG 78]

I always q tine my fish with about 50/50 success with pet shop fish. The ones that don't make it seem to always have similar symptoms before they die. They are good for 2-3 days then develop a red looking sore on their side. Then 2 days later they die. I usually use sea cure for ich and use api medicines for fin rot and for this red sore. I'm just wondering if anyone has some insight to this as I usually pick out fish, hold them for about a week at the store make sure they eat, and are good looking fish. I've lost a powder brown tang, potters angel, and the last one was a nice copperband butterfly. Any help is appreciated