Are you sure its hair algae and not dinoflagellets? Are you using RO/DI. If so have you checked TDS level. Water changes? How often. What type of lighting do you have? T5 and power compacts lose spectrum and have been known to promote algae growth. Just throwing fish and other organisms in your tank that consume algae is not a long term solution. A yellow tang does not belong in a 47 gallon tank. You have a nutrient issue in your tank. Manually remove as much of it as you can, shorten your light cycle, and stay on top of your water quality. turbo snails work. If you want something more exotic and fun to watch check out tuxedo urchins.
