В /etc/shells добавьте /usr/local/bin/fish:
# List of acceptable shells for chpass(1).
# Ftpd will not allow users to connect who are not using
# one of these shells.
/bin/bash
/bin/csh
/bin/ksh
/bin/sh
/bin/tcsh
/bin/zsh
/usr/local/bin/fish
Затем chsh -s /usr/local/bin/fish.