Leviticus 11:9

And these you may eat, of all that are in the waters: every one that hath fins and scales in the seas and the rivers,

fins and scales following the traditional Aramaic rendition of the words.

SNAPIR V’KASKESES’ (FINS AND SCALES). “Snapirim — these are what [the fish] swim with. Kaskeses — these are the scales attached to [the body of the fish].” This is Rashi’s language, and so also it is found in the Gemara of Tractate Chullin. But you should not [be led to] understand from their language that these scales are really [inflexibly] fixed to their bodies and attached to the skin of the fish [so that they do not come off at all]. Rather, they are called “fixed” because they do not move to and fro from the fish, nor do they quiver as do the fins. They are the round coverings, the plates of which are like finger-nails, and they can be removed from the skin of the fish by hand or knife. But those [scales] which are fixed and attached to the skin of the fish and cannot be removed from the skin at all, are not kaskeses (scales), and the fish [which has them] is prohibited food. It is for this reason that the Sages in the Gemara, 66 b. See in Exodus, Seder Bo, p. 132, for meaning of the word “Gemara.” said that “Kaskeses is the cover of the fish.” In the words of the Tosephta: “The kaskasim are those [objects] which cover the fish, and the snapirim are those with which the fish moves.” This is also the translation of Onkelos, who rendered the word kaskeses as klippin, [which means literally “shells”], for they are the “shells” of the fish which can be taken off and peeled away like the bark of trees and the peel of fruits. Such is also the meaning of the verse stating, and he [Goliath] was clad with a coat of ‘kaskasim’, for all their [war-] coats were made of rings, and some people made “scales” in them in order to close up the openings of the rings, so that thin arrows should not be able to penetrate them, and it was those “scales” that were called kaskasim. The Sages also mentioned them in the Gemara [Tractate Sanhedrin] in the Chapter Cheilek: “Dressed in coats of mail made with scales.” Now Jonathan the son of Uziel translated [in the above-mentioned verse concerning Goliath that he was clad with] “a coat of galbin,” this being of the expression, the razor of ‘hagalabim,’ which are “leather workers.” [By so translating, Jonathan thus] intended to say that the covering at the openings of the rings in this coat of mail was like that of the scales on a skin of a fish, as they used to boil hard skin and cover the coats of armor with them, a practice they do to this day. Understand this.
The reason why fins and scales [are signs of permissibility as food] is that those fish which have them always dwell in the upper clear waters, and they are sustained through the air that enters there. Therefore their bodies contain a certain amount of heat which counteracts the abundance of moistness [of the waters], just as wool, hair and nails function in man and beast. Those fish which have no fins and scales always dwell in the lower turbid waters, and due to the great abundance of moistness and gatherings of water there, they cannot repel anything. Hence they are creatures of cold fluid,, Note 264. which cleaves to them and is therefore more easily able to cause death, and it [the cold fluid] does in fact cause death in some waters, such as stagnant lakes.