Cowfish are members of the Boxfish family; These fish have a rigid body made up of bony plates covered with a sensitive skin that can be damaged by cleanerfish.
Most are poisonious, releasing a poison into the water when threatened in the confines of a tank, this often proves fatal to both to the boxfish and to the other fishes.
These fish relish worm foods but will try anything.
All in all I do not recommend these fish for anything other then their own tank and by experienced fish keepers. I have had a couple of bad experiences with Box/Cowfish and would never have another one.