-this is peculiar as the back end swelling just looks like they do before they have to essentially crap out the waste...something seems to be either blocking it or preventing them from properly evacuating the tail end of their digestive system. the bulges look normal to me from what i have seen in healthy, hungry, and well-fed YOY LNGs. it could be that the water changes were constipating them...just in terms of balance with water chemistry, which can sometimes be a problem. at this point i wouldn't tweak the water any...even with all the guppies, unless you have a ton dying, they won't all put out enough biowaste to equal one large fish, so i would cut the water change volume and frequency. for now, i wouldn't do anything.
you didn't say whether or not your water parameters were normal...what were they at last test? even if everything looks right you may have a problem. young LNGs are fragile, amongst the most fragile in the family (CBGs also pretty testy). keep us posted, but for now i wouldn't do anything too drastic. i wouldn't add any more food either...let them deplete this supply and fast for about a day and see if anything improves. with the dead ones, did you cut them open to see if a lot of waste had built up? this seems like something is constipating them more than some other infection, but again, tough to say with such fragile fish. i hope these guys pull through; keep us updated--
--solomon