Commentaire sur L’Exode 7:10

לתנין means A SERPENT.

ויעשו כן כאשר צוה, They did as G'd had commanded, etc. The reason the Torah writes the words כן כאשר, i.e. repeating basically the same word is, because it tells us that Moses and Aaron a) did what G'd had told them to, b) did not do so until Pharaoh had requested the miracle from them.

A serpent. The term תנין when on land means a serpent (i.e., a snake). And when in the sea it means a [kind of] fish. So too does the Radak explain תנין .

And it became a tanin In the wilderness, which is a place of snakes, the staff was turned into a snake; but in Mitzrayim, which is a place of taninim, it was turned into a tanin. A tanin [תנין] is a "crocodile" in this language, like in Yechezkel 29:3 "The great crocodile [hatanim התנים] that crouches in the midst of his streams". And he didn't do the second sign in front of Par'oh, to bring his hand out from his breast and behold it was afflicted with tzara'at like snow, because if he had been afflicted with tzara'at they would have removed him from Par'oh's presence, and something like this is in Bereishit 50, that Yosef didn't have permission to come before Par'oh when he was still tamei meit before he had buried his father. [In Bereishit 50:4 Yosef lays a petition before the court of Par'oh rather than going directly to Par'oh]

The Torah also wanted us to know that though anyone who performs miracles immediately raises his own image in the eyes of the onlookers, Moses and Aaron had no such considerations. They did what they did only at the behest of G'd. It is the mark of the truly righteous not to perform their own will but to perform what they perceive to be G'd's will.