I purchased a pair of H Reidi's and placed them in a ready tank.
These horses fed on frozen mysis when I picked them up, but when I put them in the tank, they wouldn't touch the frozen mysis.
I soon discovered they were after the mysids hiding in the rock, and, althought there is no sign of mysids remaining, (check at night time with a red filter over a flashlight lens) the pair wouldn't eat frozen mysis, brine, blood worms, or anything I try, until yesterday when one of them has been seen to pick at two or three each feeding. The other hasn't begun yet but I'm hopeful.
It certainly didn't take much to get them off their feed for frozen foods, but it's a chore to get them back on again.