Seeing the previous advice you recived I desided to do a little reaserch on your behaf..
This seems like it could very well the begining of a Myxosporean or Myxidium leei Parasite Infestation.. in the advanced stages the eyes are sunken and dark, you see a bony ridge around the eyes and the gut becomes bloated.. once the parisites have made it to this stage the likelyhood of saving the fish is slim.. there is nearly a 100% loss in aquaculture fasilities that become infected (of course being able to catch it early in these fasilities may be more difficult then it is at home)
page 59-62 of this document has some rather heady information about it..
after the death of many fishs they do autopsys to determan the cause of death.. in wich case these seem to be in the gut.. that would make me want to treat this as an internal parasite infestation.. hopefully someone else can coment on how they would proceed.. but I would start a regiment of internal parisite foods as per instructions on the container..
anyone have a recomended anti-parisite food?
I do know jungle makes one and is likely the easiest to find...
I realize your peramiters are considered "fine" but.. in this case.. I would do some water changes to get the NO3
consentration to below 5ppm if it were my tank.. for the time being..
I hope this helps..