Be REALLY careful going this route, but yeah, they do sell erythromycin powder at most aquarium stores to treat topical infections on aquarium fish. I have no idea how that shit is dosed. Have a back story just in case someone asks. Like "I was just at my mom's house and noticed one of her gouramis has a bacterial infection on its side. It's not fungus and it's definitely not ick."

For antibiotics I'd rather try an UrgentCare or something first. It is counterproductive to take antibiotics without the correct dosage and schedule. It can lead to antibiotic resistance, which is very bad (because then the solution for infection becomes amputation).