I noticed the koi pop in and out of existence on visible parts of the screen - it would be nice if incoming koi swam into frame from offscreen, and outgoing koi swam offscreen before disappearing.

Yes, that was very jarring. I didn't bother reading the intro text that said the other fish are other people on the site, so I just thought this was janky crap. The creator shouldn't assume people read anything. I see a button that says "Enter", I click - I'm not reading anything else. So it's going to cause a lot of confusion when I arrive in a pond filled with way too many fish for a zen-like experience, and even more annoyance at the constant flickering in and out.