May I suggest that if this is an "antibiotic" as you are describing, you may want to use it even longer than 3-4 days or longer than "until you see results" idea. Antibiotics need to obliterate the bacteria in the fish, not just begin the process. What you may set yourself up for by only treating until you see results is a recurrence of the disease only with a stronger bacteria that is resistant to antibiotic therapy. This idea is the same as if you were to take an antibiotic for a bacterial infection, just start feeling better, quit the antibiotic, then allow the bacteria causing the infection to "figure out" the drug and evolve to a new strain which is more deadly.