If it is Hexamita, aka "Hole in the head", you can try feeding medicated food and medicating the tank.
You could use this food for a pre-made option-
There's several metronidazole options for water dosing, but I use SeaChem MetroPlex.
This is somewhere to start. It's difficult to make any kind of diagnosis from the information we have here. Step up water changes, dose a little bit of the Stress Coat daily until things get better, and good luck.
Some reading on Hexamita -