You can feed the fish antibiotics if they're still eating instead of dosing the whole tank. I use the seachem meds. You can wet pellets with antibiotics (kanaplex) and use 'focus' to bind them together if available.
I did this for a week and then a 2ppm dose of levamisole in the tank to boost the fishes immunity.
I've used Kanamycin and Erythromycin to get rid of BGA. No problems for the plants.
Theoretically, I don't think antibiotics would hurt plants. They might have a secondary effect in killing filter bacteria and soil bacteria. Thus, I would clean filter before treatment and change water after treatment. I'm sure that you know this.
Sorry about your fish. If you don't have a UV sterilizing filter, if ever there was a time to use it, I would consider doing it now. It would at least stop the contagion.
I had a personal experience described in my article 'Mycobacteriosis in Aquarium Fish'. I have used three different brands, so don't get too mired in the details. I didn't. If the UV light bulb has been used less than 6 months, it should drastically reduce bacteria in the water. I think a UV sterilizer is particularly useful in a quarantine tank for new fish (so vulnerable!) or for stopping a contagion.