They are medications.
A product called "Prazipro" has Praziquantel in it. You might be able to find that at your local petfish store. If not, you can order it at many places Online.
Levamisole might be impossible to find locally. There are some places that sell it on the Internet though. There are a few listed here-
http://www.google.com/products?q=Levami ... tnG=Search
OR You might want to use Flubendazole instead. I don't have any experience with this medication though, but there is some information on it in the levamisole article that I gave a hot link to earlier.
I wouldn't do the treatments at the same time.
A Levamisole treatment is only 1-2 days, then followed with a few large water changes (like 2 -50% changes). Then a Prazipro treatment would be for 1 week. You can use the antibiotics during and after the treatments.
Before Quarantining the loach in question, do a few decent water changes on the main tank, about 30% each, so that the skinny loach will acclimate better to the q-tank water, when it gets those large water changes.
When setting up the q-tank use about 70% of the water from the main tank. Make sure the q-tank water is kept warm and is extra well filtered. You might want to use a well established filter from the main tank for this. It's important that toxic ammonia or nitrites aren't allowed to build up. Give the loach a secure dark hiding spot, like a cave, it ease the stress.
Try to feed the loach a little frozen bloodworm, shrimp, or clams during the treatments. Start feeding just a little at a time and try to build up the portion size a little each day and hopefully the his appetite will increase and he'll start bulking up.
Keep in mind this is a long term process. It could take the loach 2-3 months to bulk up. Even after he bulks up, relapse is possible.
You'll probably want to treat the main tank with the Prazipro and Levamisole also. The reason to separate the skinny fish from the rest is so that he won't have to compete for food for awhile.