Well, you need to know what is causing the hair algae in the first place.
What do you phosphates read at?
Have you attempted to remove as much as you can manually?
Have you increased water changes?
Tell us more about your tank (fish, food, CUC, equipment, size)
Keep in mind that not all fish will eat hair algae, I've had 2 lawnmower blennies and one ate it, one had no interest.