I've kept the heniocus in my avatar in a 120, but only for about three years. At that point they're about 4-4.5" nose to tail. They need room to streak but they spend a tremendous amount of time picking at the rock which requires no space. So they need plenty of rock as well as open patches. This fish is currently in a 90 until I get the chance to catch it and place it in a 5' 100 I recently set up.
They will eventually grow to 6-7". They eat a TON! So, they poop a ton. Bioload wise, they are another tang. And they can turn on your zoas and fleshy LPS without warning. I suspect mine eats aptaisia too ... but IDK? But, I do not have any aps and I know they've been introduced on incomming corals.
Long term ... they get pretty big and need a bigger (180ish) tank.
All in all a beautiful fish in an SPS tank with the room.