The fish attack in a specific pattern - when two are killed, three more spawn, then two more when those three are killed. The pattern repeats for a minute of real time, and once all the fish are killed after the minute mark, the Rhizopas spawns. Defeating the Opinicus Fish is not difficult at all, but since they respawn over and over the best course of action is to kill all but one of them, then wait for a minute to pass and then kill it to fight the Rhizopas.