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.