HEY! you live in lincoln, my hometown. (Go huskers)
Actually, Nitrofurazone is a anti-microbal, its suppose to kill fungus and microbes, not Gram negative bacteria.
If the Furan didn't cure it, then it may well be columnaris.
Best thing you can do is find some "Kanaplex" or kanamycin sulfate antibiotic to go along with the furan. I know furan claims to kill columnaris and the other two gram negative bacteria, but it has never worked for me, it only kills microbes/mold and gram positive bacteria.
Let us know more about them and you can not waste money, possibly save them and make an informed buy not just guessing, which everyone hates.
Edit: Sorry didnt realize there was a page two. Primafix and just keeping the water clean by vacuming and doing religious PWC should be enough to keep fungus at bay, another thing you could try is as my fellow cornhusker stated Methylene Blue or Malachite Green. Honestly ive personally never had much luck with furan, but some people swear by it.