Might be best to stay with what you have- five might be about the aggression/competition limit for a 150. Six may be feasible if they were all brought through quarantine together and added at the same time (or maybe three now, three later), but a new face in the midst of five established ones might end very badly for the newcomer. And you run the risk of disease and infection if the fighting gets bad. I'm setting up a 180 right now, and though there's about 11 tangs that I want, I'm going to go with three, maybe four. If you really want another, perhaps you could try using an acclimation box for a few days when you introduce him, along with a few mirrors around your tank. It may also help to feed your established tangs very heavily if introducing another one; sending them into a food coma on seaweed may mellow them a bit.