Sunil was concerned that giving antibiotics to his fish might make them more susceptible to infection, and they very likely could.
In humans the normal flora of the GI tract and other parts of the body play a very important role in protecting the body against infection, hence they are part of the "immune system", a generic term.
When you remove the beneficial bacteria in the body, you are creating a niche which pathologic organisms can exploit and colonize.
Not trying to pick nits, but indiscriminate antibiotic use can have very negative consequences.
