Frens - I am not a Doctor or other medical professional, so please regard this as information and consider your individual circumstance and conduct due diligence
JASE medical at https://jasemedical.com/ is a site in the US staffed by US licensed doctors who will provide a "prepper" supply of most commonly required prescription antibiotics. They say that the kit is meant for one person. They will do an intake assessment and based on medical information will provide a prescription for medicines. In the event of allergies to meds, they will prescribe alternates.
If you are interested, here is a video discussion with the founder
Naw, just order them. Search for "fish antibiotics".
maybe....but the govt is further restricting vet meds. Your fish should be safe but a lot of meds for our aquatic and four legged friends will be prescription only. See Dr Bones article at https://www.doomandbloom.net/the-fda-and-veterinary-antibiotics/