try a treatment with wundertonic or Methelyne Blue, both should be easily available from LFS, I personally prefer to cycle with fish so no lectures here, mouth fungus less likely to be caused by cycling problems than by poor handling during shipping/ mouth fungus is not the same as columinaris, as columinaris is a bacteria. however wundertonic should treat both potential problems. if it is only a few neons atm I would just sit back and wait and accept the odd mortlity or even euthanise the fish as it is worth about 1/2 the price of the smallest bottle of treatment (harsh but well.... true). as any treatment will slow down your cycling process quite a bit and make your experience of fish keeping into a chore instead of a hobby (plus it can stain your gravel/tank/ornaments)