I know there are a few people that have had success with multiple tangs in a 75g aquarium, but I don't recommend it. I wouldn't recommend you try a powder blue in anything less than a 6ft tank, they are very powerful swimmers that cruise the tops of the coral reefs in nature racking up the miles. For 4ft tanks I wouldn't recommend anything but zebrasoma tangs. And only 1 per tank, JMO.