Before we tackle the algae, lets find the source of the problem =)
What kind of tank are you running? Plants? or fish only?
If its plants, please tell us how much lighting you are using, your fertillizing routine, and whether or not you inject CO2.
For both kinds of tanks, we're gonna need to know what kind of inhabitants are in the tank, what kind of filtration you are using, and info on any source of light this tank may be exposed to.
There are a few algae eaters that will eat hair algae. However, the only thing that comes to mind for such a small tank are C. japonica shrimp (aka. algae eating shrimp, amano shrimp, etc.). The only problem with using shrimp for algae control is that you are going to need a horde of them, plus they only work if you can cut back on whatever is causing the algae. Shrimp work great, but only if the algae is somewhat under control.
American Flagfish will eat hair algae like noodles, and SAEs (siamese algae eaters) will eat them as well (as long as there is no other food sources around). American Flagfish may work in a 15 gallon tank, _IF_ they are the only fish in there. SAEs will outgrow a 15 gallon tank (as they can reach sizes of 6 inches).
I would like to repeat... finding the cause of the algae, and eliminating that is always better than adding algae eaters to a tank. Preventing algae before it starts doesn't add any kind of bioload to your tank, while adding an algae eater does.
*edit*
Oh, btw... I don't recall hearing about any kind of pleco, or oto that eat hair algae.
HTH
-Richer