Try enabling fish like this

nixos-config/configuration.nix

@@ -26,6 +26,11 @@ in
     };
   };
 
+  # Enable completions by nix
+  programs.fish.enable = true;
+
+  environment.shells = [ pkgs.fish ];
+
   networking.hostName = "${hostname}"; # Define your hostname.
   # Pick only one of the below networking options.
   # networking.wireless.enable = true;  # Enables wireless support via wpa_supplicant.

nixos-config/home-manager.nix

@@ -11,8 +11,6 @@ in
 
   # root home
 
-  programs.fish.enable = true;
-
   # keep everything using home manager within this block
   home-manager.users.${user} = { pkgs, ... }: {
     home.username = "${user}";