Mysis hatching question

[rayjay]

Not all mandarins will eat brine shrimp, and some will only eat ongrown juveniles so you would have to experiment first to see it yours will eat them.
If so, you could add daily hatchings to the tank, or every other day or so.
I don't know your system size but many many years ago I had my 90g with over 100 lbs rock, and running about 5 or 6 yrs before adding a mandarin.
It was great for a long time but eventually it started to get skinny and died when I'd had it just over a year.
Mine wouldn't eat brine of any size, and live copepods were not available back in those days.
(I know, I'm old)