Actually, that isn't exactly true (equating the sea hare to the sea apple.) Sea hares have no toxins in them, so the danger of them dying in your system is the same as anything dying in your system -- there will be a big spike in ammonia that your system may or may not be able to handle. Pseduocolochirus spp. actually have a toxin that they can release, called holothurin, and this toxin can kill fish. I don't know if it is 100% scientifically documented that all sea cucumber species contain this toxin, but reports seem to suggest that, even if all cucs have the ability, some are much less likely to release their toxin than others.