Since there are no longfin Bolivians - yet - it's obviously Mikrogeophagus ramirezi. The longfin version of this species tends to be very sickly, probably because they mostly come from the far east, where the Rams are all raised in an antibiotic soup to allow overcrowding. This results in fish with no resistance to diseases when they are shipped off, but also damages their normal internal bacterial load. This prevents them from being able to process food correctly, and they can die from malnutrition even when well fed. That's why so many have sunken stomachs when purchased.
I would theorize that it may be possible to re-establish the digestive system by feeding live foods. Not sure which ones would work best, though.