I wouldn't give antibiotics to a fish that isn't sick. Every time you do, you run the risk of creating antibiotic resistant strains of bacteria. Just take extra precautions to keep everything that touches their water as sterile as possible.
This happened to me once in my divided tank, actually, it was horrible. One fish suddenly dropped dead and then slowly over the course of the week I lost the other two, one that I'd had for four years and one that I'd had for less than a month. I was sad to lose one I'd had so long, and just as sad that the other never got the chance to live a full life. The antibiotics did nothing, so I assumed it was viral or some kind of super strain. I totally empathize with you.
All I could do in the end was sterilize everything with potassium permanganate and start over.
I recommend reading this article on the risks and the proper usage of antibiotics: http://edis.ifas.ufl.edu/fa084
it will give you a better idea of how antibiotics work and why "shotgunning" isn't always an appropriate response. I hope things start going better for you.