I once heard that if a fish has fins it's enough - and you don't need to find out if it had scales.
- Is this true?
- What is the source?
- Why did the Torah write fins and scales, if fins is enough of an identifying mark?
I believe you have the cases reversed. If you can tell that the fish has scales then you do not need the fins because all scaled fish have fins as well and you know the fish is kosher. The converse is not true and if you find a piece of fish which you can identify as having fins you may not eat it until you ascertain it also has scales. (Shulchan Aruch Yoreh Deah 83:3)
As Menachem explained, the Mishna in Niddah 51b states that all fish which have scales have fins, after which the Gemara asks why the Torah gave both signs if just scales would be enough. The Gemara responds that it is "to make the Torah great."
The Avnei Shoham (R' Shachor, 1894-1964) had an interesting re-interpretation of this Gemara, with major Halachic implications. He explained that the Gemara was saying, essentially, that as far as the sages of the time knew, every fish that has scales has fins, and they did not know of any fish that violated this rule. The Torah mentioned both to aggrandize the Torah and reveal its Divine authorship, in knowing that there are other fish out there which have scales and do not have fins, and therefore the Torah required both, even though to the best of the knowledge of the times just scales would be enough. Therefore, one need not be bothered by the discovery of fish that violate the rule of this Mishna, as the Gemara was really explaining that the Torah itself was aware that there are such fish out there!
[This would have major Halachic ramifications on the validity of relying on just having scales today, if there are fish out there which do not, as R' Shachor assumes there are.]
So to answer the questions according to R' Shachor:
1 - The Mishna says it, but it was only discussing the known fish at the time, and it is not universally true.
2 - Gemara Niddah 51b
3 - To show that the Torah knew there were fish out there which had scales and no fins, even though no known fish did.
The Mishna (Niddah 51B) says that "All [fish] which have scales have fins, and there are those which have fins but do not have scales."
The Gemara wonders why the Torah needs to give us two signs, if all fish which has scales have fins, and the Gemara answers "Yagdil Torah Veyadir" (To make the Torah great and glorious).
Rabbi Yossi Jacobson has a class and article on this, based on the writings of the Lubavitcher Rebbe. In it he talks about the metaphorical fins and scales of the Jewish People, progress and armor (limiting change), and how both are needed "To make Torah great".