Again, from wikipedia (http://en.wikipedia.org/wiki/Longhorn_cowfish):

If severely stressed, this species may be able to exude deadly toxin, ostracitoxin, an ichthyotoxic, hemolytic, heat-stable, non-dialyzable, non-protein poison in the mucous secretions of their skin. It is apparently unique among known fish poisons; it is toxic to boxfish and resembles red tide and sea cucumber toxins in general properties.
All boxfish carry the toxin -- just because you haven't seen posts about it doesn't change that... As beautiful of fish as they are, IMO, they belong in the sea.

-Corruption