You nailed one of the more debated topics. IMO I wouldn't keep one in my 125gal, based on their swimming patterns. To compare, I do have a Naso in my 125, which most will tell you is quite cramped (and would never recommend), but after studying the way the fish swims, I felt better with a Naso then a Hippo. The Naso is a much more deliberate, 'cruiser' so to speak, as where a Hippo tang generally swims 'excitedly.' Much more sporadically, franatic, and usually at a much faster clip. The Naso will eventually dwarf the Hippo, so I'll be looking for a new tank in a bit.