Romet B is a brand that is recommended by the goldfish gurus on the net, although I have not found it locally. It is best to read the ingredient in the med. For goldies, you should avoid tetracyclines (most bugs are resistent), erythromycin is recommended although there are others. I have used Jungle's anti-bacterial food (a combination of a Sulfonamide & nitroflurazone) as I cannot find erythromycin food except in 1 Lb tubs (for pond fish). For bad infections (like septecemia) you might also add Jungle's anti-parasite food, which has metronidazole .... a good antibiotic for gram negatives.
There is no great risk in giving an antibiotics (some use it routinely in QT). However, if you use it, you need to do it for 10-14 days (even if the fish is better) or you risk selecting out resistent super-bugs. Personally, I try not to over-use antibiotics. If it looks minor, I would rather watch & wait.
A 5 gal is rather too small for a 3" goldie. If you must QT, it would be better to have at least 10 gal - even a cheap rubbermaid tub will do. Goldies will foul a 5 gal quickely, and the resulting bad water might be worse than the original disease.