I didn't notice any difference in my tangs going from a 90 to a 180, although they now have room to get up to fragging speed.
But, bigger is always better, it gives more room for fish and corals to grow. 90 to 125 isn't a huge jump in volume, but does give you an extra couple of feet to add more coral. Lots of 180g tanks up for sale lately at really good prices, maybe go for a bigger upgrade
