Kuhli Loach Super Skinny

[OrigamiOokami]

I've had a set of six Kuhli loaches in my 35 gallon planted tank for a couple months now. They've all grown and seem pretty healthy, aside from one. He doesn't seem to have gained any weight, and looks like he's starving--but he's been with me for months now, and I assume if he was starving he'd be dead by now...and likewise his cohort are all plump, so I'm not sure what's wrong. About a month ago I treated for internal parasites, just in case, but there's been no weight gain in that time. Also, no aggressive tank mates, ample hiding.

Parameters:
35 gal, 24" deep
Ammonia/Nitrite: 0
Nitrate: 10ppm
pH: 7.4
GH: 2
KH: 3

[Diana]

There are several different internal parasites. Do a little research and figure out which ones your medicine should have taken care of.
Then assume it might be the other type of parasite and get a different medicine that targets that pest.

[OrigamiOokami]

There are several different internal parasites. Do a little research and figure out which ones your medicine should have taken care of.
Then assume it might be the other type of parasite and get a different medicine that targets that pest.

Thanks! I'm having some trouble figuring out what other type of internal parasite it could be. I'm currently dosing Prazipro because it seemed well reviewed and safe, but do you know any other medicines I should use? I was also thinking Levamisole, since it seems safe as well.

No other fish in the tank have shown signs of infection in all this time, though, do you think it's still most likely internal parasites that just aren't able to jump to a new host?

Thanks!