How long have you had the flame? It could be a victim of cyanide poisoning. Generally anywhere from 2 weeks to a few months they will be fine and then the effects of the cyanide take over and literally causing the fish to digest it's own innerds once the stomach is compromised. Here's a copy/paste from a post of reefrunners
Direct flights make export quick and a high percentage of cyanide caught fish survive shipment although they only have an estimated five weeks to live. External detection of the method is impossible. However, the fish have sustained internal damage which soon takes it's toll. This includes damage to the spleen and the kidneys & liver (followed by bacterial infections). The stomach lining is lacerated releasing digestive enzymes into the gastro-intestinal tract (basically the fish digests it's own intestines). The gills are inwardly damaged causing oxygen restriction and blindness linked to brain damage. Cell renewal is inhibited in all the organs mentioned except the brain which of course cannot renew cells. This results in rapid deterioration- in other words cells are lost more quickly than they are renewed.
In aquarium centres most of these doomed fish look fine. Two signs to look for are firstly, erratic looping movements or continuous, repetitive nervous circling and secondly, a suppressed appetite. This is in fact their temporary reprieve; which is why it is important (ONCE YOU HAVE DETERMINED THAT A FISHES ORIGIN IS NOT SUSPECT) to only purchase full bellied fish which are accepting food. Often it is the cyanide caught fish's first meal that triggers their ultimate demise. Their damaged kidneys and liver are unable to perform detoxification and the fish is poisoned by it's own wastes.
Barring that, I would suggest posting all your water parametrs and food choices in hopes we may be able to narrow this down. As well as the what other fish are in the tank.