they are not aggressive eaters... if you feed a little extra that sinks he will eat that and try mysis as well, it is healthier. also enrich the food with vita chem and garlic... i feed mine brine and mysis enriched in my biocube and he is fat and happy, i will have had him 4 months soon...
if it eats or shows interest in brine in the store grab it while you can and continue feeding it that in your tank. mine eats brine and mysis and i try to feed it every day. if it does not get fead prepared food the day of he eats the copeopods.