Salt baths will be just fine wit the medicine.
When was the last time you broke the tank down? undergravel filters need to be cleaned periodically, in 20 gallons or more, once a year is good, in under 20 gallons every 6 months.
Do not wash your gravel as you do not want the tank to have to cycle again. You can rinse it in some tank water though. I would wash the tank out with hot water and same for the decorations as if you are just setting it up new.
What size is your tank and ho often do you do water changes?