Thats weird that the zoo and aquirium wouldnt take it. One of my old silvers is over three feet. And lives in the boston aqauirium. I would say cut its head of like we do to the fish we ketch fishing. Filet and skin it cut it up into small peices freeze it. And feed it to your other fish. Cant get much better fish food then that if you have been trying to get rid of it for this long and no one will take it. What else can you really do with it. I mean even if you post it on here for free if some one pays for shipping how could you being a regular hobbyist not a vendor. Ship such a large fish. I mean maybe you could but i couldn't. Good luck with your decision it would break my heart to kill one of my fish. I love my fish but if you cant hose it any more and cant get rid of it. There really isnt any other choice in my opinion.