Yeah, you would have to be extremely lucky for the random_system to contain the fleet you want to destroy. Try limiting the random system to the right one(s)
Code:
random_system = {
limit = {
any_ship_in_system = {
fleet = {
has_fleet_flag = colony_tankers_fleet
}
}
}
every_fleet_in_system = {
limit = {
has_fleet_flag = colony_tankers_fleet
}
destroy_fleet = this # not sure how to ref the fleet here
}
}