Mine would not eat without the sand, he was much happier when I added it in QT.
I had PVC for him to hide in, and I did an 8 hour light cycle after 2 weeks to promote diatom growth for him to eat. When I added the sand at week 3 he began to eat nori. Also found out he really liked TDO pellets.
If you have a clear bottom, I would recomend doing something to make it opaque. Either a thin layer of sand, or painted.
I was so scared mine was going to starve, just be patient, keep food available, and while I know it is not recomended to put in calcarius rock, i put 2 in that totalled a little under 1lb. Anything to make some ditritus and diatoms/algae available.
While algae seems to be their main diet, they also pick at detritus and meat based pellets.
Definitely try the TDO ones, idk what it is about the larger TDO, but the instant I put it in his tank the first time he went berserk eating it.
They will also most likely be very shy for a while, mine hid as much as he could until week 6.
Edit: I did Jay's recommended QT, wirked fine, waited 6 days for copper to start. If you do add sand and rock while treating copper, get a hannah copper checker. Mine never dropped much, but it definitely soaks some up and releases it later.