well in a nano tank i wouldnt recommend a sea hare. I had one and luckily i noticed my skimate was black and saw that the thing was dead. A nano tank would be DONE from all the toxins if that thing wasnt caught. The crab ive had before and it didnt bother any corals that i know of, but ive heard that they eat sps? and a Urchin would knock down everything in its way like AJ said. For something that ONLY eats algaes its not good in a nano cause you cant produce that amount of algae needed for him to live longer than the time it takes him to eat all the algae. Is there anything else like a certain snail or hermit that eats hair algae and also eats food, other algaes, detritus w.e something that it can still live after killing the hair algae.