The fish actually don't slow down performance that much, it's a pretty lengthy explanation but here's a quick summary:
There are multiple ways to draw a 3D model to the screen, when the devs just started they didn't know as many of the quirks to 3D rendering (graphics and other visual stuff), and they made the game in one of the not-so optimized ways, but recently they've begun to use a method of 3D model drawing called Instance Rendering, which they use for farm wheat, fish, pigs, wolves, birds, and many other things. Instance Rendering works really really well when you have a massive amount of models to render, even if they move around a lot, and doesn't mess up performance that much either, I believe the devs are slowly migrating some of the older things they made to Instance Rendering, peasants and buildings, for example, don't use Instance Rendering when they could, relatively easily, do so. Long story short, fish aren't too much of an issue.