Forgive me if I’m posting too much–I’m still new and do not know the etiquette. But, I did just find this scholia concerning the Phaedrus and I am really excited 
Hermias, In Platonis Phaedrum scholia, p. 32, line 9
Ἀχελῷος δέ ἐστιν ὁ ἔφορος θεὸς τῆς ποτίμου δυνάμεως τοῦ ὕδατος· διὰ
γὰρ τοῦ μεγίστου τούτου ποταμοῦ τὸν ἔφορον θεὸν δηλοῦσι τοῦ ποτίμου
ὕδατος.
Acheloios is the chief god of the rivers and the power of the waters. This is because(?)
he is the largest of the rivers, the overseer of the (river) deities and they are manifested in the water of rivers.
I think you have a bunch of details wrong. I’m not entirely confident but I would render.
Acheloos is the god who oversees the drinkable quality of water. For through this largest river they show him to be the god who oversees drinkable water.
Oh I see—I was translating potimou as rivers, hence my confusion.
Thanks very much for the correction.
I thought τούτου referred to Acheloios. “For by means of this [god’s] largest river they signify the god who oversees fresh/drinkable water.”
I find the attributive position of τούτου a little baffling, but yeah I think you might be right.
Maybe τουτου = υδατος ποτιμου? “by means of the greatest river of this [fresh water]”?