Furan 2 is probably the easiest to get from an LFS or big box store since it's API. I figured you were doing water changes already since you're running a hospital tank which is great! But doing one prior to treatment will most likely give the most optimal conditions as bacteria multiplies very rapidly. Hope the fish pulls through. Also, sorry for the loss on ur other guy. I've seen those red patches before on fish with some kind of toxic reaction from ammonia, bacterial infection, and medication overdose. Believe me I've made all those mistakes in this hobby.