To follow up, I found a page all about levimasole at loachesonline. It had a rundown on places where you can purchase it. The big hangup for aquarists getting access to it is that it is a great antiparasitic for internal worms and is topically absorbed BUT it is sold as a product for sheep, pigs, and cattle. The only ways to get it for fish are to buy a medicated fish flake sold by Jungle where ymmv, or to buy the pure product packets for pigs and use the dosing guideline at loachesonline, or documentation written by a gent named Charles Harrison (google "inkmaker fish harrison").
This excellent medication used to be available for aquarists in pure form (I'm told) as a medication made by the now out-of-business "aquatronics".
Discus breeders are apparently aware of all this and use levimasole to deworm their fish, despite its lesser-known existence, because it works darn well and covers a very wide variety of internal parasites that no commercially available medications will (except for the flakes that offer limited results -- if the fish don't eat it, they don't get the medication).
Maybe someday we will see this reappear as a commercially available medication for aquarium use, but until then the burden is on the aquarist to figure this out and to obtain the product through unfamiliar channels such as the farming website where I purchased it today.
On the upside, the product is pretty affordable at around 18 dollars plus shipping! That's less than many aquarium medications you can buy at your local pet/fish shop. The medication is said to be filter safe *and* plant safe and healthy fish can be treated along side the infected ones if quarantining isn't possible. Additionally the medication is reported to help boost immune responses in fish.
On the downside, you have to follow the strict dosing guidelines--no mistakes, or you will kill your fish instead of help them. Make no mistake though, if your fish have internal worms there is a nearly complete mortality rate if the fish is not treated, and the parasite can quickly spread to everything in the tank. The medication is said to be harmful to most invertibrates (snails and shrimp for example).
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