So I was trying out the browser version of the game after spending some time on 1.2.5 to see how well it plays and literally during my first 3 minutes of playtime I came across the bonefish easter egg.
From what I can tell the Easter egg seems to randomly trigger upon entering and exiting the aquarium. Though it's rare and it only shows up once per save file meaning once you encounter it you have to delete your save file if you want to see it again. I think it's meant to be mysterious because it's not in the fish index thingy and there's no star attached to it.
I have another image that proves that I encountered it on the browser version of the game. Here you go:
