A little input please. :)

[fishoholic]

I've had two emperor angels in both cases it took over a year for a 2" juvi to change into full adult colouration, within this year they grow to around 5-6". Emperor angels get quite large (over 12") and will need a minimum of a 180g tank as they start to reach adulthood, and a even larger tank as they continue to grow.

IMO a 20g is way to small of a tank for an emperor, even for a short period of time. If your koran is fighting that much with it odds are that wont change over time, and most likely would get worse as they get larger/older. Your best bet is to take the emperor back to the LFS where you got it from and explain to them that your angels do not get along and ask for a refund.