English: Whatever has scales has fins, but there are some that have fins and do not have scales.
On the daf / in context: The Gemara records the empirical rule that scales imply fins, so that in practice the presence of scales is the decisive sign of a clean fish.
Type of Learning
Empirical rule tied to the verse (Niddah / Chullin) — the Torah’s two signs are reduced in practice by observed correlation.
What Is Learned
Any fish that possesses scales is kosher; the absence of scales (even if fins are present) renders it unclean.