That says 18" maximum length!?
There was one at my LFS that grew very fast. The had trouble finding a place for him because of his size and rapid growth.The Long Horned Cowfish, Lactoria cornuta, is also known as the Longhorn Cowfish, Yellow Boxfish, or Polka Dot Cowfish. This fish is a variety of Boxfish with characteristic "horns" protruding from its head resembling a bull or cow. Though in its juvenile stage, the "horns" are yet not...www.saltwaterfish.com
This says 20” in wild, 10” in aquariums. Who knows.......
I don’t believe they are fast growers, and at least for me, that is not the thing giving me reservations