For logging purposes: This fish appeared to display the same symptoms as the last one that died. The difference being the last one was in the process of recovering from a severe ich infestation. Noted lack of appetite, thinning, red protrusions in the mouth* but still normal activity. No hiding, no wounds, lesions, abnormal spots, HITH, rot, or anything else to suggest any sort of infection. Near death the fish was arched and breathing heavily, mostly unable to swim. Both fish displayed the same body curving. I don't know what the cause of it is yet.
I do think I have an idea though. To me, the fish look like they are "wasting". Simply refusing to eat, and becoming more and more malnourished until they pass away. I think something similar to hexemia is to blame. I can't say for sure that I am correct though because they never exhibited and pitting in the head.
* On the red protrusions: Ive asked about this a couple times and not gotten much of a response, but I have been able to figure it out on my own. I think.
I have noticed, after very close watching, the other pandas also have red teeth. When they are not eating, the teeth slip back in to a space behind their lips and you cant see them. It looks very similar to how a sharks teeth function. Out when biting, hidden the rest of the time. This makes sense as the forward teeth would make it much easier to rip plants apart to be chewed by the pharyngeal teeth. I think that as the fish was losing weight, the fat deposits around the head were also diminishing causing the lips to pull back slightly and expose the teeth.