Well I coulnd find any concrete info on them releasing toxins if they die but I did find this.......Most sea hares tend to protect themselves by the exudation of a bright purplish ink when they are alarmed. There remains some debate about the exact function of the ink, but research suggests that the purplish ink is a concentrated secretion of defensive compounds isolated from their algal food (primarily fleshy red or green algae or cyanobacteria, depending on the species). The best data support the hypothesis that the ink functions for defensive purposes. Observed behavioral changes from potential predators included: (1) bristle erection by fireworms; (2) increased mucus production by other opisthobranchs; (3) reduced feeding behavior, increased grooming behavior, and temporary pauses in heart and scaphognathite beating by crabs; (4) reduced or increased activity by cryptic and exposed sea urchin species, respectively; and (5) rapid swimming by fish (Carefoot et al. 1999). When threatened, these animals apparently eject their defensive ink to confuse and irritate any potential predators while they make a hasty retreat. This ink is not particularly toxic (although it is certainly a stressful irritant to most species), but it is a potential hazard for pollution and stress if not removed from the aquarium. Fortunately, the ink is easily removed with the use of activated carbon, or by water changes.