The volitan does get up to 15", but they are not a constant swimmer, like a tang or angel, so they don't need a larger tank. I think the 75 is plenty big for what you have. Also your humu is one of the smaller triggers. Hopefully the size of the volitan will deter the trigger. Keep him fed well you should be fine. But I also think more rock would make it look better. Your fish don't need as much live rock as some others might, so maybe something else to fill in some space. A sunken ship or something. But yeh, nice fish too. :happyfish