I used NLS Algamax, and when it was available, HTH Super Soft Spirulina (but I believe HTH has been discontinued.
When mine first came in, they were only about 1", took 2 years to get the largest male to about 7".
I added the shoal of Guinacara, because the ornatum were at the same time, skittish, yet very aggressive with each other, and adding the slightly larger Guinacara seemed to distract, and temper the aggression. I tried lemon tetras first, but they were not a sufficient distraction.
Being a riverine species, I was doing 30% water changes every other day.