Columnaris can be treated if you catch it early. There are several strains. One of them can kill the fish within 24 hours.
Most antibiotics are safe for plants.
Columnaris is Gram negative, and Nitrifying bacteria are Gram positive and protected in a bio film, so the medicine usually does not affect the beneficial bacteria.
Some meds may affect snails or shrimp.
If you can feed the medication to the fish, this is probably the best way to treat.
Kanamycin is one medicine that is useful against Columnaris.
Other things that will help:
Columnaris is more active in tropical tanks with a temperature over 76*F. If the fish are OK with a lower temperature, then lower it a degree or so per day until it is 74-75*F.
Columnaris seems more active when there is more nitrate in the water. Do enough water changes to keep the NO3 low. Vacuum the floor of the tank and clean the filter.
If you are treating with a medicated food, you might consider adding a UV sterilizer.
Remove sick fish before they die. When they die the disease spreads from the dead fish.