a cowfish (and many other puffers and boxfish) will release toxins when they die, or get stressed, they prabably wont if theres no reason to, the toxins will wipe out the tank. I wouldnt trust a sea apple, or any other cucumber, because A: They can wipe out the tank, and B: IMO, cucumbers, and stuff like that, just kind of gross me out. Dont know why, they just do.