As far as I know, all fish that have scales have fins. So why does the Torah mention both fins and scales when requiring scales would be enough?
Actually, the Talmud (Hulin 56b) asks exactly the same question. The answer the Talmud presents is much less clear than the question. The Talmud simply says "In order to magnify and glorify the Torah" (Isiah 42;21) There are several approaches to understanding this answer: Rabbeinu Nissim says that it is in order to prevent someone making a mistake and thinking that the Hebrew word for scales in fact means fins, in which case he would mistakenly rule that non kosher fish (which have fins but no scales) as permissable. The Ritvah does not think that this mistake is reasonable, but rather that the Torah is teaching us that fins and scales are not only a means of identifying the fish, but actually cause the permissability of the fish. (They are a "Sibah" - cause - and not just a "Siman" - sign of identification). We might expound upon this by saying that fish to be kosher must have both means of locomotion and of protection from the environment which are specialized for fish. A third approach (Rabbi Moshe Sofer, the Chatam Sofer on Chulin) is that there may be a species of water creature, not a fish, that has scales but not fins, and that the Torah is eliminating that species. We would have difficulty supporting that interpretation.