While Peachey & Courtney are correct - what someone really means when they say it is: "Bye! Also, I am a nerd. If you recognize this reference to a science fiction/humor novel, then we probably like the same tv shows, books, and movies and we can be great friends! If you don't know what I'm talking about, we can still be friends - but we might not have the same tastes in things."

It's a quote from the Hitchhikers Guide to the Galaxy book. The book is called "So Long and Thanks for the Fish" and references a message that the dolphins left when they had all left Earth.

"So long and thanks for all the fish." This is from Douglas Adams, who wrote "The Hitchhiker's Guide to the Galaxy" series (your quotation is the title of one of the books). You should read the series to get all the nuances, but the basic context is that all the dolphins departed earth (because they had superior intelligence) and left this message for humans. "Thanks for all the fish" refers to the aquatic park dolphin shows.