First of all it is "too small" not "to small". Secondly let's keep it on track for the OP here. If we start discussing how big the 150 is for the tangs, then I would say with the three tangs he has he is pretty much already done ... LOL
If it is not big for a hippo it is not big for a naso either.
Alas, people would comment on 150 being small for tangs without looking what they house in a much smaller tank themselves...
So to the OP. Sorry about this. It is all how you interpret the things.
I have tried the mirror trick with success. Try to put a vanity mirror in the tank and see if anything changes. The PB or GR might just get busy with its own mirror image.
Good luck.
Sunny