If it is a drug resistant bacteria then it is eventually fatal.
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It's a good idea to clean the tank before you re-treat the aquarium with medication. If you have to treat every couple of days, wipe the inside of the glass and do a huge (80-90%) water change and complete gravel clean, then add the next dose of medication.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
If you have to treat the tank every day, try to clean it every day before re-treating or at least do it every second day before re-treating.
Cleaning the tank before adding the next dose of medication removes the biofilm and dead organisms (bacteria, fungus, etc) from the tank so the medication can keep working on the fish.