hi
we used to hav neons a few years ago & lost a couple who displayed the symptoms you describe, they ended up being hugely obese & very swollen, they still ate,swam around,bickered with their tankmates but always looked grossly swollen around their bellies. i tried feeding daphinia,peas,bloodworm everything i could think of that may have helped cure constipation but nothing made any difference, the fish carried on like this for a few months before dying. i posted with a pic on another forum but nobody knew what the prob was.....i'll see if i can find the pic of my fat neons for you to see if yours are thesame.unfortunately i cant offer any advice as to what to do to help the fish.sorry