Few fish eat directly unicellular algae or other unicellular protists, which are...

Few fish eat directly unicellular algae or other unicellular protists, which are indeed the main producers of DHA and EPA.

Those unicellular living beings are normally eaten by small animals, e.g. copepods, which are eaten by small fish, which are eaten by big fish.

Thus all of them get the required DHA and EPA.