Meaning of "where all fish souls end when they go back to the greater scheme of things"?

Hello everyone. I am reading a novel, and I came across this expression. Could you please let me know its meaning?

I wondered what kind of evil monster she turned into when her love died—did she tell you it was finished: Just let it go? Did she drop you back into the fish tank where you sank or swam, or did she release a few bubbles at a time and throw you tiny pellets of food as she did with Inky that night at the party, so you wouldn’t go belly-up, though you know and she knows it’s only a matter of time before they pick you up and flush you down where all fish souls end when they go back to the greater scheme of things? Was I making all this up, or was I myself gradually being put in a straitjacket before being dunked in a pickle jar as I looked up at the hole that was about to close on me?

- André Aciman, Eight White Nights, Third Night

This is a novel published in the United States of America in 2010. This novel is narrated by the nameless male protagonist who meets Clara at a Christmas party in Manhattan. Now the protagonist is thinking how Clara would turn into a monster after her love is over.

In this part, I wonder what the underlined expression means.

What would "greater scheme of things" mean? I vaguely guess it might be another way to say "heaven" or "afterlife", but I am not sure of its meaning... Thank you very much for your help.