I have snorkeled with them a number of ties in cenotes in Mexico, and it seems the majority of their diet in nature is algae and detritus, with an occasional insect that falls in the water, and a live bearer fry. They constantly try to eat the thousands of livebearers that surround them, but very seldom succeed, it they make 1 out of every 10 attempt, I believe that is optimistic.
So I totally agree with cichlidfish that quality pellets are a great choice, and many times, beauty comes with age, and lots of water changes.
below some video I took last Nov
https://www.youtube.com/watch?v=e8OVy3JKxcw
https://www.youtube.com/watch?v=txmi1mng_XQ