I'm not sure ... partly because I don't know boxfish medical sensitivities, and partly because Hydroplex is a "proprietary blend" that specifically does _not_ include copper, formaldehyde, malachite green .... or a list of other known ingredients effective against marine pathogens.
(If the above makes it sound as though I'm suspicious that Hydroplex might be snake-oil steeped in magic beans, well ... so be it.)