It's not fish science
Making creatures swim like fish is suprisingly difficult. It leads right into the territory of non-holonomic locomotion. Equations with lots of funny greek symbols.
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The reason behind all this is that fish are much better at swimming forward than in any other direction. They also need some velocity to make a turn. In that sense fish are like higher dimensional cars. Now stir some bumping into each other and colliding with terrain into the mix. Add a pinch of non-uniform drag and two scoops of underwater currents. Now you got yourself a real problem at hand. Enjoy!