Let's look at it from a standpoint of WPG, which is a pretty sound method to determine lighting over a midsized tank such as your 55Gal.
A 55 gal tank is 48" wide.
Nova Extreme T5HOx2 | Current What's Next
A 2 bulb T5HO fixture (like model 1126 in the link above) is 108 watts. This equates to about 2 WPG of T5HO lighting. I would agree that this on the verge of needing CO2 for algae control--although I bet there are people who would run this tank with just a little fert supplementation and no CO2.
But -- the fixture could be ran with 1 plant bulb, and 1 actinic bulb. This would make it a low light tank. You would have several benefits from this: The single plant T5HO bulb would provide a good amount of usable light to grow low light plants (much more usable light than a single tube T8). The actinic would give the water a great blue shading, making fish and plants "pop" under the light. It would also leave room for expansion to run both T5HO bulbs as plant bulbs in the future, should you decide you want to run CO2 and grow some more difficult plants. If you intend to update lighting on a tank, I would rather spend a little extra up front and get something that leaves you options for expansion, rather than buy 1 low powered fixture now, and then down the road go out and buy another fixture.
