How do you humanely euthanize a very large 12 inch x 6 inch Oscar fish?

I am not sure but it is nice to know your not just flushing it,,,contact a vets office and just tell them you need to know I am sure they will tell you, or if you have a Zoo around you call them they will hwlp you,,,Good Luck and sorry :( about your fish

Background on Oscar: He's 11 - 13 years old, and had a serious case of hole in the head disease, we treated it however believe that it has driven him crazy. When we walk by the tank he swims up and bashs his head on the tank cover, the sides, onto the bottom of his tank and is doing this many times an hour. This has just started happening yesterday. He has seriously injured himself and I believe he is in alot of pain, however since he is so big we do not know how to euthanize him. We have a blanket covering the side of his tank and this has helped keep him calm.