"Black Areas of Death", those are the souls of all the fish that have died , so technically when you run your fish into the wall their soul is being joined into the void....
lol, I don't really know I did not have time to polish that, I was going to add sea-weed, coral, and crabs walking around, but I had no time, the whole void idea seems very intresting tho, as for the theme all the fish are joined together (in a school), and all the other fish join your school.