Mouth problem
 

my partner has a group of phantom tetra, theyr mixed sex, and are very nice fish, however one seems to have the bottom of its mouth missing! it looks perfectly health, water conditions are fine, but where the bottom of its mouth should be theres just a hole, which it appears to suck food in through.

Any ideas what could be wrong, it is very difficult to get a photo as it likes to hide away, or is always moving. but i will try to get a snap if it helps

Can we see a picture please, also, what other fish are in the tank?

Tried my best to get a snap, the one in the back is the fish in question, the front one is a normal one