I don't use antibiotic, so I can't help you with that...sorry...
If the fish is struggling to swim, eat, in general I look at quality of life-if he still act and be a fish then I do everything I can, but when they stop having any quality of life....I end their suffering humanly....a big difference with being alive and having quality of life....however, on the same note....you also don't want to give up too soon, only you can decide how long you want to try.....
I am sorry to keep bothering you, but my fish is floating at the top on his side but is still living. Have you ever tried ampicillin? How do I know if he is suffering? he is not laboring with the breathing, but he is really bloated. he still eats and i put in the epsom salt. He is fighting his butt off to stay alive, so I feel I should do more.. am I getting carried away?
I wouldn't think so, usually once the symptom of drospy (edema) shows up to the point of scales sticking up it is usually fatal with 5-7 days more due to system failure and kidney shutting down. So if he has survived for 3 weeks I would say it is bacterial in nature-look for an antibiotic that covers/treats both gram negative and gram positive bacteria. The Epsom salt can be helpful for the symptom of dropsy (edema).
I use Epsom salt 1tsp/gal up to 3tsp/gal (not aquarium salt) for no longer than 10 days along with daily 100% water changes
I keep them in the Epsom salt 24/7 a treatment not a bath/dip
I pre-mix my Epsom salt 1tsp/gal in a clean 1g jug to make dosage and water changes easier.