I'm sorry to burst you're bubble and maybe I'm wrong as I don't keep saltwater fish but I don't think a private fish keeper can keep a sardine in captivity. From what I know these are pelagic schooling fish meaning they need a lot of swimming room and like to be in large groups. I also believe that they are filter feeders. Any sort of filter feeding creature should be left in the wild or to public aquariums.
Rule number one: Always research a fish before you buy it.
Rule number two: Always cycle your tank.