I have a 65 gallon tank, and when I had a massive hair algae problem after a combination of getting new corals just before leaving for a week (and having my brother in law overfeed the tank), a sea hare was the only thing that could takkle the algae bloom. In 3 days, it had mowed down over 75% of it... a week later, it was all gone...problem was...a couple weeks after that, I couldn't find the sea hare (I assume he starved).
I was pre-warned of this, and had bee putting large pieces of letuce in, but I don't think it helped. All in all, very cool creatures. Sometimes they hide in the sand and look exactly like the tops of small rocks!