I'll put a word in because no one else seems to want to, but some people wont agree with this; I would dose the main tank both pimafix and melafix.
Though it will be a pain in that size tank. The products are entirely plant safe, as the label says, which has shown to be true in my own and other's tanks. They've worked well for me in the past, and are really quite well made to work in conjunction with one another. They're natural remedies too, so that's both good and bad.. Bad being mainly that they aren't very strong against resistive strains and are more inclined to really supplement healing.
The dosage should be limited if there are any scaleless fish, but it doens't look like you have any.. In general these products have really only been helpful to me when the infection/fugus/damage or whichever is new and fairly minor..
However, I would think it would be easier to quarantine the angel if I didn't have any fears that whatever the infection is had already spread to other fish... If you did separate and quarantine any of the fish, a stronger antibiotic could be used..
What's the reason for this outbreak? New additions to the tank?
Edit: there are also homemade versions of the above mentioned medications, a quick google should tell you how people make them