Stupid fish store

I just started keeping saltwater fish.  I the past I kept fresh water cichards, Oscars, disc etc.  I thought I would move up and start keeping saltwater fish since the new nano movement has shown the fish world that it can be done.  Well, I got the tank going, and a few damsels (just to see if I could keep them alive) all was going well.  So I got a mandarin goby.  The LFS said he would eat froze foods, which he is not!  Now I fear the fish will kill over.  Such a shame.  I am going to contact another fish place in Cherryville and I hope he can help!