The SW fish that the Nurse Shark would most likely be part of the natural prey in the wild - is the following - Mullets, Flounders, Spots, Croakers, Drums, & herring.
Mackeral, Salmon, Tuna & Trout might be okay as an alternative.
For small juvenile Nurses - it's better to do small feedings 5-6 times a week - as Zoodiver suggested. But keep the total amount you feed the juvenile shark to about 10% of total body weight per week. If you go much higher than that, you will likely see the shark's growth rate rapidly increase - to as much as 2" per month (24" per year).