I agree having all the same fish for the entire session (if it is a long one) is a bit restrictive. But having fish change every time they leave the screen, in my opionion, would look weird!

I have an alternative suggestion.

Maybe it would be possible to include a "refresh random fish every n minutes" option, where you set the number of minutes in the settings. When it gets to the set time, the aquarium fades out, the fish are replaced, and the new aquarium fades back in.

What do you think ?