I would euphanize that fish immediately. The problem your going to have with medicating the tank is your currently in the beginning phase of a fish in cycle which needs daily water changes, thus the problem the medications need time to work and you'll be battling daily water changes with daily medications. I'm afraid this could be a tough battle here.
I prefer Fungus Clear by Jungle over the Mardel products, I had a nasty strain that ripped though a tank two years ago killing 16 mbuna before the Fungus Clear worked. I tried Maracyn 1&2 and Maroxy with zero impact on my strain.
Drop your tank temperature down into the low 70's slowly overnight to help slow the infection, then on top of the medications dose the tank with aquarium salt at 1 tablespoon per 5g and keep doing this everytime you do a water change(only replace the salt you removed).
While the above is a probable outcome, you might want to try one other option: A Quarantine tank.
If you set up a small tank, like a 10 gal, with good clean water, a heater and an airstone, you won't have a cycling problem, can leave the fish in the meds for the recommended amount of time and might save the fish. Since you won't be feeding the fish during the treatment (or at least drastically reducing the feeding), the ammonia level in the quarantine tank will be negligible by the time you are supposed to do a water change during the treatment. This will give you time to get your main tank to a 0 ammonia level.
As for meds, If this is in fact columnaris, it's a bacteria and not a fungus and my preferred antibiotic med for this is Nitrofurazone which is found in Fungus cure. I prefer Furan 2 however because it attacks both gram + and gram - bacterias and columnaris is a gram - bacteria.
Keep these other factors in mind:
"Because the bacteria thrive on
organic wastes, the potential for columnaris outbreaks can be controlled by regular water changes and tank maintenance, including vacuuming of the gravel. Proper diet and maintaining good water quality in general will keep the fish from being stressed and therefore more susceptible to infection. Placing new fish, and promptly moving any sick fish to a quarantine tank will help prevent the introduction and spread of the disease.
To avoid spreading the bacterium to other tanks, nets, specimen containers, and other aquarium equipment should be disinfected before each use. "
Hope this helps...
