Complex sentence: Oscars will often lay claim to an area of the aquarium and will get very aggressive towards other fish encroaching on their newly established territory inside the aquarium or lake, the size of the territory varies on the size and aggressiveness of the fish based on its surroundings.
Split into simpler sentences: Oscars will often lay claim to an area of the aquarium and will be very aggressive towards other fish encroaching on their newly established territory inside the aquarium or lake. The size of the territory varies depending on the size and aggressiveness of the fish based on its surroundings.