I fish would not live for years without excreting......no more that
you would. Sounds like dropsy to me. The fish could poop while your
getting the mail, taking a shower, eating, etc. When you vacuum the
gravel, do you see any remmants of poop? If not, look at your filter
for dark brown slimy stuff. Put the fish in a tank without gravel
(like someone mentioned) and then you can clear your mind.