Midrash su Levitico 11:11

1) (Vayikra 11:9): ("This you may eat of all that is in the waters: All that have fins and scales in the water, in the seas and in the rivers — those may you eat.") From its being written "you may eat" what has (fins and scales), I understand not to eat what does not have (fins and scales); and from its being written (Vayikra 11:11) not to eat what does not have, I understand that I may eat what does have. Why was this repeated? To make them subject to (transgression of) a positive commandment and a negative commandment. "You may eat of all that is in the waters.": For I might think that since (small fish found in basins [as opposed to fish found in running streams]) are permitted explicitly (even without the signs of cleanliness) and by implication (I might think that) just as explicitly they are permitted only in basins, so by implication they are permitted only in basins — Whence would I derive that even if they are found in holes, pits, or caves, he may bend down and drink from them without apprehension? From ("You may eat of all) that is in the waters."

9) (Vayikra 11:11): "And detestable shall they be to you": to include their admixtures (if an unclean fish became mixed up with clean fish and cannot be recognized). How much (of the unclean fish) must be in it (the admixture) to be forbidden? The weight of ten zuz in Judah, which are ten selaim in the Galil, in a keg which contains two sa'ah. The brine of an unclean fish is forbidden. R. Yehudah says: (How much of the unclean must be in the clean to forbid the brine?) A revi'ith (of a log of unclean fish) in two sa'ah (of clean fish). R. Yossi says: One in sixteen.

10) "Of their flesh you shall not eat": (The exhortation) does not apply to their bones and to their fins. "and their carcass shall you hold in detestation": to include midges filtered (from their native element).