I do love the hints that Sephiroth has acquired himself some predatory fish as well as the more normal guppies and goldfish and such. That does suit his personality. I also like the hints that Zack hasn't really noticed this (for any number of reasons, such as exhaustion, lack of familiarity with breeds of fish etc).
I really enjoy the dynamic between the two of them you've been creating throughout this series - of Sephiroth gradually feeling his way toward humanity, while from Zack's perspective it's rather like luring out a tiger. I agree that fish are a good pet for Sephiroth.