If you haven't euthanized yet, have you tried Melafix (tea tree oil)? I had a cichlids that was beat to a pulp by a tank mate (male), and couldn't swim, and it brought her back.
Well, Jannika hopefully I won't have to test this out anytime soon, but I'll keep it in mind the next time I do have to euthanize a fish to see how it goes.