“Shellfish” is the secret name that the fandom uses for an IRC channel. As for the fandom itself, I can explain certain aspects of it that you need in particular. We also have a wiki, if you want that.
It started around the time that snapshots for the Caves and Cliffs update were being released. People joked about how the raw copper sprite looked like raw salmon. I made a parody of an Among Us intro scene (“Raw Copper: There is one Raw Salmon among us”), and Copper Fish was created. Later on, the creator of Copper Fish turns all the minerals into fish, and I suggest giving them roles.
That’s a brief history of the fandom before it started expanding off of Scratch.