Adding fish repository
echo 'deb http://download.opensuse.org/repositories/shells:/fish:/release:/2/Debian_9.0/ /' > /etc/apt/sources.list.d/shells:fish:release:2.list
then adding vender's key to subscribe updates
curl -O https://download.opensuse.org/repositories/shells:fish:release:2/Debian_9.0/Release.key
apt-key add - < Release.key
then installing fish with apt
apt update
apt install fish