here is a web page with Durban water temps (these are surface water temps)I doubt your gem tang will be happy at that low temp even if Kingi would be
eThekwini Online - Beaches Home Page
look at the depth range of a gem tang
http://www.fishbase.org/summary/speciessummary.php?id=7631
look at the depth range of a kingi angel
http://www.fishbase.org/Summary/SpeciesSummary.php?id=25426