If you've tried the fenbenazole route and your worms showed immunity go to Aquabid for levimosal if you are US. Searching around I found that people have gotten levimosal from MVP on aqua bid and from Charles Harris off his own site. From word of mouth both seem legit. I am ordering mine from MVP because his price is reasonable and I have heard good reviews on him. May not need it but want it on hand JIC it comes back. Some others stuff I came across (depending where ppl where) was getting fen Ben from vets in powder or liquid form, flubel 15, and sera nemol( want to say uk). I have no experience with any of these and double check my spelling on the drugs as you google. In mean time you can try to buy your fish time, frozen daphnia is a decent laxative to keep them from getting blocked up, I use it because my fish won't touch peas. Also keeping high water quality and feeding high quality food (in limited amount as to not block them up) may give them the strength to muscle through. Fish that are too bad off won't survive treatment so euthanasia may be your best bet. The most humane way I know is Clove oil. Easy to obtain (tooth are isle if drug stores) and doesn't take much. There is a great thread on I believe Oscar fish that comes up when you google how to euthanize a fish with clove oil. As long as your fish are still eating, IMO, you still have a chance of pulling them through with the right meds. Once they stop eating treatment options get limited.