Sorry to hear this. Bacterial infections are very hard to gauge, especially considering we have no good way of treating them in fish. Dosing antibiotics into the water can be slow acting, biodegradation can occur, antibiotics can have harsh side effects, etc. For really bad infections you almost have to perform daily antiseptic (e.g. acriflavine) baths to keep the infection under control until the antibiotics kick in. Also, it is important to note that antibiotics, in and of themselves, do not cure a fish. Antibiotics merely control the population growth of bacteria in a fish long enough for its immune system to eliminate them. So, a fish with a compromised immune system is doomed no matter what you do. :(
It has been suggested that injecting a fish with antibiotics or applying a topical antibiotic would be more effective, and I certainly agree with that. However, in most cases a sedative would be needed beforehand to avoid damaging the fish. The first part of the video below shows how MS-222 can be used for this purpose: