O ay, you’re like the fisher-folk, the men who hunt for eels,
Who when the mere is still and clear catch nothing for their creels,
But when they rout the mud about and stir it up and down,
‘Tis then they do; and so do you when you perturb the town.

[ὅπερ γὰρ οἱ τὰς ἐγχέλεις θηρώμενοι πέπονθας.
ὅταν μὲν ἡ λίμνη καταστῇ, λαμβάνουσιν οὐδέν·
ἐὰν δ᾽ ἄνω τε καὶ κάτω τὸν βόρβορον κυκῶσιν,
αἱροῦσι· καὶ σὺ λαμβάνεις, ἢν τὴν πόλιν ταράττῃς.]

Aristophanes (c. 450-c. 388 BC) Athenian comedic playwright

The Knights [Ἱππεῖς], ll. 864-67 [Sausage Seller] (424 BC) [tr. Rogers (1924)]
    (Source)

Speaking of demagogues like Paphlagonian (Cleon).

Alt. trans.:

• "You are like the fishers for eels; in still waters they catch nothing, but if they thoroughly stir up the slime, their fishing is good; in the same way it's only in troublous times that you line your pockets." [tr. O'Neill (1938)]
• "For you are circumstanced like those who fish for eels. When the lake is still, they catch nothing; but if they stir the mud up and down, they take. And you catch, if you disturb the city." [tr. Hickie (1853)]