I think there is something called "mouth rot" that can cause the mouth to stay open. My Malawian Eye Biters got it if I remember correctly, but that was about 14 years ago. There is medication for it, but they all died. I always wonder if they got it from all the store bought feeder guppies I fed them, because I kept their aquarium in immaculate condition. Ever since then I've been very weary of feeding store bought feeders to my fish.
I'm not saying that's what your fish has, just throwing another possibility out there. Good luck, get well Texan.