What's wrong with my Neontetra? Please help. Looks like something in his mouth.

[CuriousCat]

Neontetra seems to be isolated in a corner. Whitish substance in his mouth and having spasms like movement.

 

[Rocky998]

Neontetra seems to be isolated in a corner. Whitish substance in his mouth and having spasms like movement.

Could be a fungus but @Byron would know better...

You could try salt as a remedy at a dose of 2 tablespoons per 5 gallons. Add it slowly so you don't put the fish in shock and do not keep it at that level for more than four weeks.
When treatment is over, slowly bring down salt levels so you do not shock the fish.

The "spasms" are probably from it being irritated and trying to itch that spot. It probably doesn't feel good I'd imagine...

 

[Byron]

I have so little experience (fortunately) with fish diseases that I would only be guessing, and that is no help. We have members here with more knowledge in this area, I'm sure one or more of them will help.

I would not start adding salt or any other medications until the issue is fairly certain. Adding any of these will stress all the fish, and if it doesn't do any good, they could all weaken for nothing.

 

[GaryE]

I've seen this. The mouth hangs open, and, sadly, the fish dies. I've taken it to be a growth in the throat, but I think it spreads. It's a new fish, right?

 

[DoubleDutch]

Personally I think this probably is the same as the often seen lumbs on the mouth of neons but now an internal one. The general thought / agreement is these might be viral cause it is untreatable, not that lethal and quite species specific .