Pristine water quality. Highest quality food: LRS or make your own fish "smoothie" with frozen fish cubes: mysis, chopped clams, cyclops, seaweed, reef mix, etc. Take one of each and place into a tupperware. Let the cubes "slump" and then pour off excess liquid. Add some vitamin. Zoecon is formulated to help fish like angels and butterflies to supplement for missing sponges, and other natural foods they feed upon. The mix will stay good for several days in your fridge.
But as far as bacterial infections goes, it's always a good idea for reefers with angels, butterflies, tangs, to keep NFG powder on hand. It works wonders on those "red sores" that these types of fish develop.