Bacterial infection can present in so many ways. Sometimes there are areas of redness under the skin, as if deep under the skin, or sometimes redness that appears to be in the outer layer of the skin, with scales slightly lifted.
Sometimes you may see a dull redness to the skin, with what appears to be points, as in pus pockets, that are trying to come to a head, forming a pimple or boil. Some that rupture and some never do, though the fish may succomb to it in either case if the right antibiotic is not used.
Some bacterial infections present with areas that have a dull flat look, or gray look, to the scales, as compared to the other parts of the body, there can be areas that have lesions that have a cottony appearance, like wet cotton, sometimes you may see ulcers or eroded areas.
An infection that has gotten into the bloodstream can cause septicemia; symptoms are redness at the base of the fins, red streaks in the fins, red blotches under the skin, thin threadlike streaks in the body.
It is often difficult to know just what antibiotic would be best. If we could ID the pathogen, microscopically, we could know generally what might be best. Even then, the best method would be to test for sensitivity to an antibiotic in vitro.
It helps to be very familiar with the appearance of your fish when healthy, and scrutinize their appearance and behavior for changes every day. Sometimes the first signs will be mere changes in behavior, hiding, lurking in a corner or behind the heater, hanging out near the surface. Those can be signs of parasitic infestations as well, you just have to be observant.