I have noticed that when you set the fish in the aquarium while it is in the bag, it will stick around the area that it was introduced to until it is used to the aquarium. So, I would introduce all the fish to the exact same spot within 10 minutes of releasing each species. This way, once the fish get used to the aquarium and decide to explore, they won't invade each others territory.
Other than that, just let them sit in the aquarium for some time and there is your beautiful aquarium!