The same as columnaris. I have a neon tetra that has a large white ball on its mouth and it has grown larger. best treatment advice for this fish would be great.

NB first tropical tank and fish put in when not fully cycled am currently treating tank with seachem prime to control ammonia nitrites and nitrates. No lectures please would do differently next time but for now it's the fish that I am concerned about. Thanks in advance.

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)

Thanks for the quick reply I will defiantly go to lfs tomorrow and get some wonder tonic as suggested thank you so much for the advice.

Mouth fungus is a bacteria and needs to be treated with an antibiotic. It is also known as saddleback. Wundertonic may work as it contains ingredients that act as an an antibiotic. If it does not work try furan.

oops, my bad :facepalm: this is what happens when I stop dealing with fish on a daily basis and fill my head full of other stuff.

This is very often seen with neons nowadays. Earlier thought it to be a bacteria, but it appears to be untreatable and non-contagious to other species (in most cases). After a lot of threads about exactly this i've come to the conclusion this often must be Lymphocystis, caused by a virus (Irido) that's only found at neons and cardinals (probably Black Widows). It's untreatable, but can disappear on it''s own. Only lethal when organs are infected. White mouthulcers are common all over the world (Holland. UK, America, NZ aso). Must have to do with pressurebreeding in Asia (as Dwarf Gourami Disease = another Iridovirus).

Cheers Aad