Cories are possible, in my early days of fish keeping I took a petshops "advice", apparently they are "pond fish" and can live in cold water (I lived in the otaki gorge in kapiti so it got quite cold) I started with five and had three left after 2 years. They were never very active and never grew. But I doubt there would be large numbers in lake taupo (maybe a couple of released fish) but they need warm water to breed and I doubt anyone would dump 20-30 (sorry :nilly: ) Cories.