Can you post a pic of them dude? The problem with them looking skinny is it can be any one of a few different diseases that take different drugs to cure. Loaches are also more susceptible to drugs than a lot of other fish so dosage is usually a bit less.
Prazi is the most commonly used/available drug for worms and flukes and works great, this is what I would try first.
Levamisole is also supposed to be great for a broad range of worms/bacterial diseases but I have heard a few bad things about using it with loaches.
Metronidizole is the best drug for bacterial diseases but is really hard to get a hold of, vets won't usually write a prescription without seeing the infected fish first.
I would personally be inclined to treat the whole tank as it doesn't take long for disease to spread from my understanding.