Reddit usually gets upset at this one. I'll just echo what most have said antibiotics aren't a magical cure all and you need to watch dosing and application closely. If at all possible you should just rely on modern medical care and advice.
That said antibiotics aren't magical. You can buy fish and veterinary antibiotics and they are functionally the same. I'd be less wary of say equine antibiotics (I've known a few rowdy rodeo guys use them and they seem to work) as fish aren't that sensitive so there may be a lack of quality. Probably not too much though because I thought I read somewhere that the majority of antibiotics comes from a handful of approved producers so the only real difference is packaging and QC. In a long term emergency scenario I think it is worth having as an option. Kinda like a tourniquet though if you need to use it then your chances aren't all that great anyway.