Question

what is the main food of marine fish

Answer 1

Krill. Krill are small marine crustaceans, usually found in concentrations in cold arctic waters. This high protein fish food is considered to be "rich" and should be fed sparingly to fish. Available in freeze-dried flakes and pellets, this fish food is used as a supplement to a well-balanced diet.

Answer 2

planktons which are of two type phytoplankton
zooplankton