Seeing yours makes me try again, perhaps starting with a dozen... With the last few I had success with frozen bloodworms - they took to it after a week or so with me, then started taking fish pieces in a few months on their own. I was so glad. At about a foot+, they developed some kind of disease, spinning out of control or not being able to swim, only float due to an air pocket inside or some such. It sucked.
The ones I tried before just wouldn't feed, no matter what I offered, and eventually starved to death. It's horrible to watch.
Have you noted any effect of the bottom on coloration? And also lighting? I keep mine pretty dim all the time and wonder if this is also why they are so dark and the pattern is unresolved. Here they are:
https://www.monsterfishkeepers.com/...retrack-and-3-black-spot-2-in-240-gal.698735/