To follow the movements of a single fish is a task which might be easy to fulfill with a camera and tracking software; it is not so easy if you try it yourself - but it trains you concentration. Below some attempts to follow the vertical and horizontal movements of a selected fish.
hunting Daphnia in a glasbowl
reflection (there are only two fish)
microorgansims at the bottom of the aquarium
Still from a video clip: feeding fish with midge fly larvae while a computer voice is reading the publications (over 1500) dealing with zebrafish from 2019 listed in the ZFIN (Zebrafish Information Network).