It depends on what species of boxfish you're talking about. Some of boxfishes (i.e spotted boxfish and longhorn cowfish) are more likely to release ostracitoxin than others like cubicus or thornback. I've had all of them (except the spotted one) before, and even under very stressful conditions they never released toxin(at least it seemed to me!). I also heard that some of them have less potent toxin than others. If you think about getting a boxfish, then go with thorback. They're the most uniquely shaped fish, and they don't get as big as other cousins. I have one right now, and it's full of personality.
