There are a few Tangs that can do smaller (in the 60-75 ballpark), but none will work long term in less. I believe it is more an issue with what is good for them, more than their ability to survive. There are a lot of sites out there that show info on various fish...blueZoo, vivid aquariums, live aquaria.com You will see differences in suggested sizes (Liveaquaria tends to go larger than the others), but it should provide a safe ballpark for required tank sizes.