Im late to this thread and from reading it, its got alot of possible issues.
I would try kanamycin with metro as its the most effective combination we have for internal infections in fish that are not eating and its the most rapidly absorbed ime.
The question is whats the base cause. Like the others I think its a bacterial infection. Its hard to say where it came from but a possible scenario is the fish was initially sick with an external bacterial infection that resolved as they often do with new fish.
Its possible the levamisole killed worms in the fishes organs and that led to bacterial infection. If the worms are in the intestines they are passed out with waste...but in organs they can decompose and lead to infections and decomposition issues. This is one reason why I deworm when I first get fish... if fish have extensive worms deworming can be a real problem.
Its a guess in the worms but I think the best suggestion is the antibiotic .. whether its from decomposing worms or some other bacteria issue. I don't think this one is one we can ID the cause on... I would suggest a lab for that.
hth,
Al