'Skinny Disease' is a term that is applied to what amounts to 'Chronic Wasting Syndrome'. Fish are initially eating, and behaving normally, but losing weight, and eventually, left untreated, it leads to death.
Many wild caught botiine species can develop it. There is no one, definitive cause or test for it.
Intestinal parasites are a likely cause in many cases. In fact, it has been suggested that one should
assume internal parasites in all wild caught fish since they are endemic all over the world...treating all wild caught fish with 2 rounds of levamisole is a very good idea. Parasites are not like bacterial infections. You don't usually 'see' any signs of them until they have become damaging to the fish. A fish can look fine and be harboring a load of intestinal parasites that will only manifest observably after they have reached critical mass and overcome the fish.
In other fish, it is a combination of intestinal parasites that have opportunistically proliferated in a weakened fish and created a subsequent bacterial infection in damaged tissues.
Other types of wild caught fish have also shown similar symptoms. Intestinal parasites that have overgrown their host will consume what food the fish intakes and the fish will appear to waste away. They can also damage the gut or internal organs to the point where the fish cannot recover, even if treated. By the time a parasitic infection becomes noticable, there has someitmes been too much damage done internally to heal.
Wild caught fish generally live with a wide variety of parasites that remain balanced by their effective immune systems. However, they have gone through serious stress in the catching, shipping, and re-acclimation process by the time they get to our tanks. If their immune system becomes compromised the usual 'bugs' that are normally kept in balance take advantage of the weakened state of the fish.
Levamisole is the best treatment for nematodes, roundworms, nodular worms and thread worms. (Trichostrongylus, Cooperia, Nematodius, Bunostomum, Oesophagostomum and Chabertia - intestinal worms) in fish. Combine that with a good broad spectrum antibiotic (only if necessary) and most fish with early signs of skinny will recover. Because levamisole is also an immunostimulator, it will help the fish fight off secondary infections if they have not progressed too far. So wait on antibiotic treatment unless you see some continuing signs of bacterial illness like redness, cloudy eyes, bloating, spinal abnormalities (fish TB) or obviously abnormal behaviors (lethargy, swirling, inability to remain upright in the water).
You are very fortunate that you haven't run into any wasting fish! It is a good thing.
And which loaches is it most prominent in? It seems the clown is one of them, I keep seeing threads asking if their clown has skinny disease in them. 
