Seem to have multiple tank illness issues?

We just returned from a five-day vacation. All our fish are usually fed twice daily, but were only fed once in the past week, and so were without food for 2 two-day intervals. I noticed today that one of our swordtails in our 55-gallon seems to have white mouth fungus, and an angelfish in our 40-gallon has fin rot. Additionally, a gourami in the 40-gallon has a red, translucent protrusion around one of his gills. I have done water changes and treated with anti-fungal/bacterials today, but I was curious - could the sudden change in feeding patterns have stressed the fish out, making them vulnerable to these maladies? (All fish have been quarantined). Both tanks have been running for several months, and we do weekly 25% water changes; pH is stable; nitrite and NH4+ are zero and nitrates are always low. Any other advice or ideas? Thanks!