Hi Kim. I'm sorry I haven't been able to check on things today, I've had some health things going on myself that kept me away from the computer until just now.*
Kivstev's advice is spot on. I didn't mention Kanamycin because I thought it might not be readily available and tetracycline is easy to find.
I wanted Kanamycin and had to order it from Petmountain.com and it took several days to get to me. I can't find it at Petsmart or Petco, at least not the ones near me, but if you can find it it would be good to use with the Furan 2.
The salt dips may sound scary, but at this point you're in "last ditch effort" mode, and I would try it for sure.
Even dosing a 1 to 3% solution of salt in a tank with Columnaris is beneficial in preventing mortality in fish, proportional to the percentage, with best results at 3%. There is evidence that it prevents infection in non-infected fish by preventing the flavobacterium from attaching to the fish.
3 teaspooons of table salt per gallon of water is a 3% solution. You don't have to use aquarium salt. Really, plain table salt is fine; based on recommendations by several AC members who have chemistry and biology degrees. The trace amount of iodine and anti-caking ingredients are negligible and won't cause a problem, and it dissolves quickly.
Use dechlorinated water that is the same temp as the tank. 3% is not terribly harsh but you could do 2% by using 2 teaspoons.
If you are using aquarium salt about 4 teaspoons is 3%, so you could make up a gallon and dilute it accordingly to make 1 to 2% if you want to be cautious.
I have one of those plastic rectangular containers that the fish stores use to dip out the fish and pour into plastic bags when they sell fish to customers. It is less stressful than netting, IMO. I use a net to kind of direct the fish into the plastic dip container, which is clear and often not even noticed by the fish.
Once you have the fish in the dip container I would carefully pour off most of it and then gently slip the fish into the container with the dissolved saltwater at the desired concentration. I haven't done dips like this for Columnaris, so I don't know how long you leave them in the solution, but I'm sure the links Kivstev gave you have more information on the procedure.
I'm so hoping your fish are better. Columnaris, which is usually accompanied by aeromonas, is a very serious infection, and often even with all one's best efforts the mortality rate can be high.
If things don't go well for you and your little friends, please know that you have made valiant efforts on their behalf, and they couldn't have had a more caring keeper. You have done your very best for them. Please know that.
Give us and update, please, we all care very much.
* I was interrupted after typing this about an hour ago, and I just came back to the computer and realized I hadn't hit the "post" button on this post.