Whether it's due to aggression, territorialism or just plain old size difference it's pretty safe to say anyone who keeps more than one non-schooling fish in a tank for a long time is going to need to separate them somewhere along the line. Now let's list the reasons for dividing fish.
There is a physical barrier which divides the aquarium fish tank into two separate areas. Slide up and down to control the barrier and define boarders allowing fish to establish different territories. Once the fish is separated players passes the level. Players must finish the level to unlock the next one, and based on his/her performance, he/she will be given a mininum of 1 or maximum of 3 stars for the level. Stars are used as a format of a ranking system. With one being for simpliy completing the level, two for finishing the level while also having a average time, and three for finishing the level in the shortest time possible.