I'd like to take a picture of the fish in the aquarium with a camera and track its location, what kind of method would you recommend?

That's exactly what I did in my graduation research.
This answer is not appropriate for the programming Q&A site stackoverflow, but it is not about programming.
Image processing is an engineering that deals with real-world things, so knowledge of real-world things, such as hardware, is more important than software.
General answers are difficult, but
For example, basic knowledge of photography techniques such as how to arrange cameras and objects, how to choose cameras and lenses, and how to illuminate.
Furthermore, if you want to observe fish, your knowledge of the habits and breeding of the fish is also important.

If this is the case, I think it's the right way to hit the light and the shape of the aquarium.