Rocephin will be a challenge to get but it may be worth it, if your bacterial cultures mandate it.

Rocephin Injectable Antibiotic Against Bacterial Infections
Rocephin Trials - by Written by Bill Morris.
Edited and replicated in Atlanta by Dr . Johnson.
The Rocephin Trials
The following was mentored, edited and replicated in Atlanta by Dr . Johnson.
Written by Bill Morris and containing extremely valuable information,this article is bona fide and professionally endorsed.

Rocephin (sterile ceftriaxone sodium) is a sterile semi-synthetic, broad spectrum cephalosporin anti-biotic that can be given either I.M. (intra-muscular) or I.P.(intra-peritoneal). I prefer I.P. route in fish as this is safer and less [potentially] scarring.
Rocephin is a white to yellowish-orange crystalline powder which is soluble in water. When diluted with sodium chloride [sterile saline 0.9%] it ranges from light yellow to amber in color. It is available in 250mg, 500mg, 1gm or 2gm vials. It is not cheap, somewhere in the neigborhood of $90.00 for a 500mg vial.

It has a shelf life of only 3-4 days after dilution when kept at room temperature. Or 10 days when refrigerated.
When Rocephin was first suggested to me, I immediately contacted Dr. Johnson. Dr Johnson recommended doing an LD50 test on several inexpensive pond comets in order to determine the upper-end toxic dosage levels.
So I acquired eight 8"-10" comets and proceeded with the testing. Before I actually began injection however, I needed to determine where I was going to start my dosage at.
A human dosage of Rocephin is recommended at 250mg for a 50kg person. This works out to 5mg/kg- (One kg = 2.2 lb). So I diluted a 500mg vial of Rocephin with 10ml of sodium chloride, which gave me a 50mg/ml injectable solution. This would mean that a 2.2lb fish (pretty large) would only need a 0.1ml injection if based on human standards. So again I contacted Dr. Johnson and he had been using some other Cephalosporins (Ceftiofur | Cefotetan) at magnified doses with success, so he recommended using 10 times the recommended human dosage for the fish with this compound. So now I had somewhere to begin testing on my eight comets.
Here is a chart on my testing and the results:

As noted there was no immediate reaction even at the higher dosage. All test subjects survived fully three weeks and are doing very well.
So after these tests it was clear that Rocephin is very safe for fish even at high dosages.
Now I needed to get my hands on some sick fish to see how effective Rocephin was going to be. Fortunatly I work at a Garden center where my full time job is taking care of fish. I get the chance quite often to make house calls, and many people bring their sick fish to me. So it was'nt long before I was able to try Rocephin on some ulcerated fish. After the first injection and waiting 2 days I began to see very good results. The ulcers were less red and more pale pink. So after 1 injection and a couple of days Rocephin was doing its job! I did continue with injections on day 4 and all the fish have healed and are doing fine.
I decided to try the injection schedule for Amikacin day 1 and day 4 because it is very cost effective for us and our customers. This way we only need to go back to the customer 1 more time other then the initial visit. Also Rocephin is not cheap! So my recommendation is to inject 2 times-day 1 and day 4 using the chart below for dosage.
Another weapon to use in the koi lovers arsenal!!
Bill Morris
Labrador Bill
Suggested Dosage chart [reviewed]:

"Rocephin, like Cefotetane is a lesser used drug, with a narrower spectrum but which is sometimes irreplaceable in the treatment of certain bacterial infections in Koi and Goldfish. This was good work done by Labrador Bill. " ~ Doc Johnson