I have seen many 12" JDs, and believe that most JDs in aquariums don't reach potential because they are held in too small tanks, where because the tanks are too small, water quality suffers compromising growth. I also believe these cichlids need enough room to get proper exercise to retain health, in tanks where they can simply turn around, and swim a few body lengths, does not offer the amount needed for robust health
In nature, in the Cenotes where I have seen the largest individuals, water welling up from below puts them in constant water change, constant current, and nitrates barely ever exceed 1ppm.
I also believe in crowded community tanks, those most aquarists try to maintain, growth inhibiting hormones remain because of too few water changes.
In nature when JDs are not the dominant cichlid, they suffer the presence of other cichlids.
In the two videos below, you can see in the first Cenote, of maybe only 10,000 gallons the uropthalmus dominate, and the small population of JDs are torn up, yet still because of constant upwelling of high water quality, they are large, 10-12".
027 zps4b102ffd
But below in a larger Cenote (millions of gallons) where JDs are the dominant cichlid, their population is robust, and much healthier.
Eden2
In nature, in the Cenotes where I have seen the largest individuals, water welling up from below puts them in constant water change, constant current, and nitrates barely ever exceed 1ppm.
I also believe in crowded community tanks, those most aquarists try to maintain, growth inhibiting hormones remain because of too few water changes.
In nature when JDs are not the dominant cichlid, they suffer the presence of other cichlids.
In the two videos below, you can see in the first Cenote, of maybe only 10,000 gallons the uropthalmus dominate, and the small population of JDs are torn up, yet still because of constant upwelling of high water quality, they are large, 10-12".
027 zps4b102ffd
But below in a larger Cenote (millions of gallons) where JDs are the dominant cichlid, their population is robust, and much healthier.
Eden2