a hippo will definately outgrow your tank. heres a little information i found for you:
The Paracanthurus hepatus grows up to 12 inches. The small size will come to you generally 1½ to 2½ inches; the medium generally 2½ to 3½ inches; the large generally 3½ to 6 inches; the XL generally 6 to 8 inches. The Blue Tang, Indo-Pacific prefers a tank of at least 80 gallons with plenty of places to hide & swim.
and:
Hippo tangs can reach about 1' maximum, generally smaller in captivity. Tank size of 55-70 gal is minimum that they should be housed in,
but they will tend to outgrow that size. A 6' long tank, is more suitable for long term care.
hth,
chris