Aristotle on Homer on eels and fish in iliad book 21

The phrase ἐγχέλυές τε καὶ ἰχθύες appears twice in Iliad Book 21, in descriptions of actions involving the river Xanthus:τὸν μὲν ἄρ’ ἐγχέλυές τε καὶ ἰχθύες ἀμφεπένοντο the eels and fish dealt with him [sc. Asteropaeus].τείροντ᾽ ἐγχέλυές τε καὶ ἰχθύες οἱ κατὰ δίνας distressed were the eels and fish beneath the eddies.The context in which these verses appear is not that important here, as this combination of words itself raised an interpretative problem in the minds of some ancient Homeric scholars: why did Homer distinguish eels and fish when eels are a kind of fish? For instance, according to Aristonicus, Aristarchus flagged both passages ‘because Homer distinguished the eels from the fish’—and Homer would not have done that, is the implication, or it is puzzling that he did so, as he must have known that eels are a kind of fish.