Most adult pairs of medium sized new world cichlids (6" to10") in nature, will set up and guard a territory about the equivalent of @ 250 gallons. In that size area, they will break off chase at about 4 ft in any direction, from a species considered an intruder cichlid , or prey, and at its territorial boundary, return to its center.
Larger cichlids (10+"), have territories that are often much larger.
Most average tanks are no where near that size, and this is why we see so many posts asking, why is there so much aggression in my community? why are my cichlids killing each other? why has my male killed his mate? why are they sick.?
After seeing this territorial behavior in nature first hand, is the reason why I set up the compromise rule for my tanks of a "minimum" 10 gallons of water per inch of adult cichlid. (L + W+ H) meaning, for example, a pair of JDs with male, 6 " L x 2" W x 2 " H, should get no less than a 100 gallon tank., with No other cichlids, but with some non-cichlid dithers.
Beside the territorial reasons, as any cichlid grows, its waste output grows with it (not only solid waste, but also and especially urine). The number and size of water changes a 2" juvie requires, are not the same for the same cichlid at 5" or 10".
Where an aquarist with a 2" juvie oscar might only need to do a 50% water change once per week, a 10" oscar might require a 50% water change every other day, especially if the tank is a size of a puddle or a rut in the road, say a 55 or a 75.
I believe this is why we see so many large cichlids turned into LFSs, scarred up form HLLE and generally unhealthy.
They live in tanks the size of a rut, and in fish urine soup.
