Midrash su Levitico 11:10

3) I might think that in respect to permitting (Vayikra 11:9), all waters were equated with seas and rivers. Whence do I derive the same in respect to forbidding (Vayikra 11:10)? From (the repetition of) "seas" (Vayikra 11:9, Vayikra 11:10) and "rivers" (Vayikra 11:9, Vayikra 11:10), creating an identity (gezeirah shavah), viz.: Just as with "seas and rivers" in respect to permitting, all waters are equated with seas and rivers, so, with "seas and rivers" in respect to what is forbidden.

4) You say that "in the waters, in the seas" comes to tell us that the waters that I permitted are the waters that I forbade. But perhaps it comes to tell us that the species that I permitted is the species that I forbade. And which species did I permit? Those which have bones and which reproduce, (which are found only in those which have fins and scales); so, I forbade only those species which have bones and which reproduce. Whence do we derive (that I forbade also) those which have bones and do not reproduce, and those which do not have bones and which do not reproduce — until you include (as forbidden) galim and frogs which grow in the water and on the land? From (Vayikra 11:10): "All that do not have, etc."

5) ("fins and scales") This tells me only of (fish with) an abundance of scales and fins, such as kiponoth. Whence do I derive the same for a fish that has only one fin and one scale? From (Vayikra 11:10) "All that do not have (lit.) a fin and a scale (kaskas)." R. Yehudah says: two scales. And even though there is no proof for this, there is an "intimation" for it, viz. (I Samuel 17:5): "and in armor of scales (kaskasim, plural) was he clothed." R. Yossi b. Dormaskita says: The leviathan is a clean fish, as it is written (Iyyov 41:7): "Its (the leviathan's) pride is the strength of shields" — these are its scales; (Iyyov 41:22): "Beneath it are sharp shards" — these are its fins.