In nature, with the water upwelling from below ground in Cenotes, puts them in almost constant 100% water change every hour., so nitrate (and all the other stuff it indicates), is undetectable (the heavy surrounding vegetation also helps suck up deleterious nutrients.

How much of the tanks water do you change weekly?
In my cichlid tanks I try to change over 100% per week, split in 3 or 4, 30% to 40% water changes every other day, to reduce nitrate and other growth inhibiting hormones.
But I also wonder about the status of the Mesoheros, ithey are so similar, if they are more dominant, they could easily be producing hormone, and attitude that hold back JD growth.
I have found JDs do not do well when housed with other cichlid species.
Even in nature, if there are other cichlids they tend to suffer that presence.
027 zps4b102ffd
In the video above, in the company of other cichlids, JDs do not really thrive.
but below where they dominate, they are much better off
Eden2