Powder Brown Tang is hard to keep in a sense that they are too sensitive on sudden parameter swing. This is true to other tangs as well. The parameter swing in smaller tank is usually intolerable to them that stress leads to ich outbreak. As long as you can keep the stress level down by maintaining good water quality, steady parameter and feeding them well they should be okay. The most critical period for them is during acclimation cause they need much longer time (3 hrs or more) to equalize parameters specially temp, pH and salinity. I had my 3 inch blue tang in a 28 gal
nano for almost 3 months, then I moved him to 60 gal
for another 6 months and now he is in my 150 gal
(72 inch long) together with my yellow and naso tangs. BTW, powder brown needs a lot of feeding to keep them happy that they get so big in a shorter time than other tangs.