how big is your tank
is it heated and filtered?
what is your water changing schedual?
what are the water parameters (nitrate, nitrite, ammonia, and ph)
does he seem bloated around the chest area?
rain water eh?... do mean like when a cloud cries? you wait for a rain storm to fill the tank? unless its some fancy distilled brand ive never heard of it. i would just recomend conditioned tap water (with dechlorinator)
and when you feed him you should really feed him pellets or flakes as a staple nearly every day of the week then blood worms occasionaly as variety/ treat to his diet. i would bet that it is the blood worms making him so ill because he really needs a well nutritioned diet
but ya more info will help us help you