faceless bloodfin: a beginner's plight

[Mephistopheles]

please help me. I'm brand new to this whole fish thing (just started my tank in July). All the fish I've got so far have been doing fine. This includes 2 platy's (Porkpie and Jaque) and a small spotted catfish (Fluff). Those fish have been there for about a month with no problems. However, I just recently bought 4 bloodfin tetra (my pet store recomended 4, but then again i'm an easy sell). Unfortunately for one of them, after bout half a week, he seems to have lost his lower jaw! I don't know exactly what happened to it. Is he sick, was it an attack, did he get stuck? He's still swimming but I haven't seen him eat and he stopped schooling with the other three. My ammonia and chlorine levels are fine but my pH is a bit high. Anyone ever heard of this or can shed light on my plight? thanks for helping a beginner in need