Where did I say that?
I was telling you that my loaches have behaved like yours and then what happened after. If you want to try other wormers you can, but I never have, for no particular reason. I don't think you should or shouldn't put the fish down, it's up to you. I haven't put mine down as they seem to just fade away quietly which seems to me to be more humane than trying to catch them in that state. But that's just me. If you think he is suffering you should do what you think is best.
I wormed my last 2 skinny loaches in the riverbed tank and they went downhill visibly in the next 24 hours and disappeared in the week after. I was advised that sometimes there are so many worms in the system that killing them is too much for the fish to cope with so they die anyway.
What has happened since I've had loaches is that I've had 6 or 7 loaches at a time and over a few months I've noticed one get skinny and die. I replace the fish and then in a few months another one goes. I haven't replaced the last 2 and so far the last 4 are all fatties but it's not to say that one will not get whatever it is. It's possible they have the worms in them all the time but if they get sick or lose condition it's enough for the worms to take hold. Unless you know someone who can look at a fish and diagnose it and treat, you can only do what you know to do and then leave the rest to nature. That's probably not what you want to hear but it just seems really difficult to get fish health perfect when we are not vets or biologists, and that they are underwater.