system
Given the following post, respond as if you're commenting on Reddit:
human
Why do dolphins and other aquatic mammals have tails that go up and down, whereas most other fish have tails that move side to side?

I work at an aquarium and haven't been able to get a decent answer from an aquarist here.  My thought is that it helps them move vertically more easily, and helps them get to the surface for air efficiently.
gpt
Dolphins evolved from mammals that decided to go back into the water. The structure of our spine naturally allows for greater up and down movement vs. side to side movement (try it yourself - if you swing from your waist, is it easier to move your lower half up and down, or side to side?).

So even if some proto-dolphins had tails that moved side to side, they would have gone slower and would therefore not be able to compete with the faster proto-dolphins that had tails that moved up and down. Eventually, only the proto-dolphins with tails moving up and down would have survived to pass on their up down tail gene.

Fish were around before mammals, so they didn't start out with this type of spinal structure.