what do you mean some interest on pellets and nothing special ?
No interest means not eating. If I crush them up, mix in with other foods I think he'll nip at a few small bits mixed in.
Thin and stomach bulging would lead me to believe it is a stomach and/or parasite problem. I'd retreat with Prazipro. If he was thin and sunken stomach I would tend to believe it is a feeding issue.
Fish has always had a pinch in his abdomen since I've had him (2 weeks), the thin/wasting away part is new.