I am not sure of the exact size of a 10g nano system. But I would imagine you could fit in a couple Clownfish which are very cool fish in any sized tank. I have 4 in my 250 gallon, there very active, bright in colour, and ever since snorkeling around different reefs in the world I have always felt acustomed to having at least a few in every tank. A great website to check out for marine fish is www.liveaquaria.com not only do they have lots of species to choose from, they give you information regarding required tank size, and full growth size as well. I would check that website out and see which fish appeal to you, and then see if they fit in your tank requirments. But as I said before I would look into Clownfish, Damsels, Cardinal Fish, ect.