Would you get your face stolen by Koh?
Aang appealed to Koh for information regarding the Moon and Ocean Spirits, Tui and La, where Koh warned the Avatar of impending danger the spirits faced. When Aang asked for their location in the mortal world, the old spirit did not provide a clear answer and merely stated that they were opposites, forever revolving.
When he told Aang that he had already met the spirits, the Face Stealer barely missed Aang's expression of epiphany when he realized that the identities of Tui and La were the koi fish in the Spirit Oasis.