heh. at first i tried feeding him fast fish. no matter what kind of fish u give him, he'll say, "Oh, a fish...mmm, yummy." and Celes always says, "Here's...a fish." I fed him all the fastest fish and it took me hours. Then all the fast fish just disappeared, no matter how many times i redid the operation.
After that, i just kill him right away. I enjoy seeing the Celes/cliff/sadness/opera music combination.