Proper size for tangs?

[whatcaneyedo]

The powder blue bullies the purple tang in my 225gal. The first few weeks were really rough and the purple would swim laps constantly, always going in the same direction and rubbing one of sides against the glass until it started to chafe. In the years that have followed they have calmed down but the powder blue still 'charges' or takes swipes at the purple from time to time. The powder blue is also a stress-case and temporarily outbreaks with a little bit of ich each time I add a new fish. This has been going on for a decade. Both tangs were medium size when I got them from well established friend's tanks that had shut down. Given this experience I wouldn't add a third to my system.