I'm unable to change my default shell to fish. If I do it, I can no longer login with the gui.
I tried it with:
https://getsol.us/articles/configuration/changing-shell/en/
sudo eopkg install fish
sudo chsh -s /usr/bin/fish $(whoami)

I had no problems with this on the budgie version.
Looks like the KDE Login somehow needs bash as default shell.
I can still login via "F2 Shell" and change default shell back to bash. Then after a reboot the gui login works again.