Baytril is my first choice injectable. Cultures are taken, usually of blood, to let me know a few days later if I've made the right choice.

Baytril Enrofloxacin Injectable Dosing
Baytril Dosing - by Dr Erik Johnson
Baytril: (Enrofloxacin) Injectible from Bayer Labs, is a 2.27% solution. I give @ 0.1 cc per 6 inches of fish, daily for three injections, then EOD for 2 more injections. Total injections, a maximum of 5.
Recommended dose, (I. ictaluri) is 14mg per kg.
I give the drug Intraperitoneally, with a 25 G needle on a tuberculin syringe.

Be careful injecting gravid (egg laden) females. The antibiotics will simply go into the egg mass and will not get diffusion throughout the body where needed! Better in those to inject IM (Intra Muscularly)
Recently, I've read a few things by people who neither keep fish nor work on them with any frequency - on a famous (notorious?) British message board.
a. Says Colin Buckinghamshire - Failed insurance agent turned Instant Koi health expert when he started importing Koi: "Baytril will not work in cold water."
Answer: Yes it will. Fish metabolism is VERY slow and the drug achieves an eventual MIC in the tissues. This MIC lasts longer than once thought, even in warm water. The bacterial progress in cold water is concurrently attenuated and so a slow rise in MIC can often intercept a slow rise in bacterial infection.

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b. According to Biff "Sandshark" Sanderson, Surfboard jockey turned instant Koi Health Expert in California when he opened his store: "Baytril doesn't work anymore."
Answer: Um. Are you practicing fish medicine? How was college? Can you back that up with cultures? I can verify, even recently, that Baytril (enrofloxacin) is HIGHLY effective against the majority of the Pseudomonas and even Proteus strains I am seeing this and over the last two years. I wish people who don't even have the qualification to be running around with needles would just SHUT UP before making blanket statements based on a fish or two.
c. According to Dwight Johns-Haughty, instant Koi Health expert by opening a message board where everyone agrees with him: "Baytril is inactivated in the liver when it passes through, so you have to inject in the front of the body."
Answer: Again, this is SO not correct and I wish people would STOP pretending to be doctors. "First pass theory" regarding portosystemic metabolism of medications administered in the back half of the body has been DISPROVEN, what; FIVE or TEN years ago? Catch up please before you look EVEN MORE like the idiots you are.