If you have the desire to change over the present biological filtration set up your tank is running on, make the transition slowly. Example: You want to get rid of the bio-canister being used and go to a wet/dry filter. Add the new filter onto the tank and run it along with the bio-canister for 2-3 weeks to allow the new filter to become "seeded" and establish a nitrifying bacteria population that will take over when the old filter is removed.
Adding medications to the aquarium. Antibiotic or antibacterial medications are designed to kill either gram-negative or gram-positive bacteria, and very often both types at one time, and since the biological nitrifying bacteria of an aquarium are also gram-negative, they are destroyed as well!
When treating fish for bacterial diseases or other illnesses where antibiotic medications are required, do so only in a QT! Matter of fact, because we do not always know "exactly" what is contained in medications or what they will kill, it is wise to treat sick livestock in a QT whenever possible to avoid putting medications into the main tank.