Caution: The Sea Hare may secrete or releases toxins that may kill fish in the aquarium. The species may release toxic compounds when stressed, or release toxic compounds when it dies. It is important to handle this species with care at all times and to never keep it with species that may attempt to harm it. It is possible for the toxic secretions to poison an entire aquarium.
Ok, sorry if I offended you. But they really need special care so that they don't release any toxins and poison your tank. Just because an LFS may say that something is fine in a tank, doesn't always mean that it is.
I just got a sea hare as well! I was having a hair algae problem and the hare was the remedy (although now I think the algae was from my own laziness and not keeping up on water changes!) Anyway, the sea hare is pretty cool, eats algae like crazy! I've been giving it sheets of seaweed and it seems to love it. I was told if there is no algae they will die. Also if they are really startled they may ink a toxin. Good luck with yours! ps how much did you pay for yours? just curious