Hello and welcome to the forum!
Tony's correct. Antibiotics will not help, and should never be administered unless you are certain they are necessary. (as with humans, needless use of antibiotics cause immunity buildup; same goes for fish, so it's wise not to use unless absolutely necessary)
Stop feeding all flake food. Give a couple deshelled peas twice a day.
What do you normally feed your goldfish? Do they have a varied diet, or mostly flake food? Fancies have eggshaped bodies, so they are prone to swim bladder issues, and flake food shouldn't be given often, if at all imho.
also, what size tank do you have? and have you checked the water readings for
ammonia-
nitrite-
nitrate lately?