And if he was to prescribe a standard antibiotic, you can buy them yourself at a pet supply. The same antibiotics used for fish are the same that you are given. Exactly the same, just different labels and no prescription required.
I used my dead dog's prescription for a sciatic disk in my back when my prescription ran out and I had a flare-up. She was prescribed the exact same medicine and dose as I was (hers was for torn ligaments in her legs I believe), and we just never threw out the medication after she died.