Mel

I have a fantail that up untill a few days ago was eating . It looks like he has developed jowls , he has no sign of infection at all . I have treated him with melafix and premafix just incase of a bacterial infection that I can’t see . If I hold his jaw and put a bit of pressure on it he can open and close it . All my water perimeters are fine I have 3 small fantails in a 55 gallon tank could someone help plz as there isn’t any I for online