Hi, thank you for the species name of these. I had some a while ago and could never get an exact discription, I only got as far as it was one of the many fw asian soles.
I found getting them feeding really tricky also, in fact a few I got just wouldn't eat (at least regularly enough). Tried live worms etc and had limited success with that. They seemed stimulated by movement, I actually had the most success with slithers of whitebait and waggling it in front of their face with tweezers.
I also had them on black substrate just like yours but I ended up moving them onto sand just because I could see they were struggling with burying themselves properly. They did seem to enjoy the even finer stuff.
Anyway good luck with them, they were a pain, but a real interesting small species. Wish I still had some.