your gonna have to post more info theres not much to go off of but good water and feed daily that could mean anything....mandarins arnt to kow for eating mysis so i would think that one would have starved.
as for the blennies if youve lost three its safe to say there also probably not eating.
in order to get more advice your going to need to post:
full water parameters
flow
feedings
tank history
tank occupants
where did you buy the fish
what did you see before they died
basically it comes down to diet,disease,bullying, or water quality/stress
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