My fish has a hole bellow it's mouth what is this?

Harlequin rasbora:

Temp:25c/77f

ph: 7.5

ammonia: 0

nitrite: 0

nitrate: 5

water is hard.

168ltr tank (39 US gallons) ehiem 300 ecco pro external filter (79 US gall turn over).

Heavely planted tank with rocks and shells. Tank inhabitance: 6 harlequin rasbora, 6 glowlight tetra, 2 corydoras habrosus (I am hoping to get another 4 if this isn't contagious).

One of my harlequins has a small hole bellow it's mouth that goes back to it's jaw. It seems to be swimming ok. It goes all the way though to the other side and you can only see it when she breaths. I'm just wondering if anyone knows if this might be hole in head as I thought that was higher up. I'm not sure if I should treat the tank with melafix and primafix. The tank has been set up since the begging of april and I haven't had any problems with the cycle and the filter has been running just fine. I was planning on adding to my cory shoal and am concerned that this might be an outbreak.