Antibiotics don't cause weakness of the immune system in fish, humans or whatever the host is for the bacteria (am a physician). Think of the fish as a house and the bacteria as rats. The antibiotic is rat poison and does not interract with the house, at least in principle, but rather interracts with the rats. This is not always strictly true, however, as we know that people and animals occasionally have allergies and other reactions to antibiotics, but these are idiosyncracies and not part of the usual game plan. See no harm coming from trying antibiotics if you can figure out a way to get them to the fish. Whether they help is another matter.
