How many bulbs are in the fixture? T6 bulbs don't come in 55w that I know of (or add up to 55w...). Are you sure its not a T5 fixture? Or a 55w Compact Fluorescent?
Assuming your tank is 20" long, you'd have 18" long T6 bulbs and those are 15 watts each. 24" T6 bulbs are 20w. Even a 24" T5 fixture would have 24w bulbs in it, I think.....so that doesn't add up either.
I'm not sure the wattage of T5 high output fixtures.