It's probably not a parasite, since you said that it is not eating like it use to and judging by the size of the stomach in your picture with the white feces hanging out, then it's probably bloat.....bloat happens when you feed the fish a good amount of food and the water is not at significant enough temperature, or a sudden decrease in water temp causing the fishes metabolism to lower and unable to process the food in the fishes stomach causing bloat....one solution is to raise the water temp up and don't feed the fish at all for couple of days to a week depending on the fish...