Commercially raised fish from the far east are often very sensitive because they are raised in huge quantities in fairly tight quarters, and treated heavily with antibiotics to prevent a stray disease from destroying thousands of valuable fish. Unfortunately this leaves them with no resistance when they are shipped out, and they just can't handle the stress of being relocated so many times.
However, these fish usually die very soon after being brought home. After 6 weeks it should have been adjusted to the conditions. What other fish are in there? I have seen some fish "ram" (excuse the pun) others in the side and kill them instantly. Did he seem to have trouble with any other fish?
I recently gave up on keeping rams. my parameters always seem good according to my apI tests... and out of nowhere they die. the first pair I had laid eggs in my tank, so I assumed they had optimum conditions for them to do so.... and within a few days they both died, without as much as loosing color.
this video is of a guy explaining why they die. maybe one day I will try again.