I'm thinking internal parasites too...especially if the other two loaches are eating and doing fine.
Of all the fish I've kept, I've most-commonly encountered this problem in platies. Their bellies would gradually get a 'pinched' (read: "skinny") appearance, and their poop was often white and stringy. In virtually all cases the fish got extremely skinny and weakened over a period of weeks, if not months, to the point of death.
IF this happens to your loach, just be sure to remove him ASAP so that the others don't feed off of his parasite-infested corpse. (Even though I lost some platies to internal parasites, the majority of my fish did NOT get infected, so don't worry about it being contagious---just make sure you remove any casualties right away).
Sorry to sound so bleak.
It is possible that something else is going on, but I'm fairly certain---based on what you've told us---that it is internal parasites.
Another thing that stands out is that your nitrates are (were?) at 100 PPM. This isn't an ideal reading...but you said you've cleaned the tank and done a water-change since then, right? If not, then do one right away.
Oh, by the way, even though your loach is skinny, is there any odd 'lumpiness' to his belly?
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