septicemia is almost always a result of poor water conditions - unstable parameters, cycling, rotting food in gravel, overfeeding, etc.
before you treat with anything, vacuum the gravel like there are demons hiding in it, then do a large (50-70% water change). perform basic filter maintenance - if you have a canister filter, thoroughly rinse the foam pads and put in new carbon; if it's a HOB, put in new carbon, etc. in most cases, the septicemia will clear up on its own once you clean the tank this way and continue with regular maintenance.
also, since many meds will kill beneficial bacteria and cause a mini-cycle or a new cycle, i'd recommend staying away from those if possible. with septicemia, it's crucial that the water stay clean and parameters stable. therefore i definitely recommend salt and heat treatment. i had great success with this method when my goldfish came down with septicemia about nine months ago. it will take a week or two for it to clear up fully, but you should see major behavior changes almost immediately after you do a thorough tank cleaning. the fish will perk up right away.
you could also start feeding an antibacterial food, as theotheragentm mentioned above. these are good for helping boost the fish's immune system and clear up the septicemia without affecting your filter bacteria colony. i have used Jungle Labs antibacterial food many times. it's fantastic.
good luck, keep us posted.