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)
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".