Could've been either the cancer or the captivity (or both). IIRC, fish do adapt their size to their surroundings, and while the Shedd tanks are large, the Pacific Ocean is infinitely larger.
He changed his gender?
This should make the gay community happy, as this is a transgendered fish. This is evidence of sex change being a natural situation.
Sometimes they say there's evidence of homosexual dogs and monkeys and such so I think this ties in with find evidence of different sexual identities in the animal kingdom