You generally can't find or put higher wattage bulbs in than the hood is rated for and tubes are a pretty set thing, so you're looking at a new hood or light fixture. If you have strip lights that sit over top of a separate glass hinged top then you might be able to just buy a second strip light but full hood type devices are more common these days.
Back to the more general topic: you don't have enough light I don't think. A 55g would need 55 watts of T-8 or T-12 (1 inch, 1.5 inch diameters respectively fluorescent tubes) just to reach one watt per US gallon and you want to get yourself up between one and two but not really all the way to two watts per gallon.
Your other problem may be that the typical 55g is a pretty tall tank unless you have a unusual shape for your 55. Tall tanks need to have better reflectors in the hood behind the fluorescent tube so that the light will focus properly down on the plants at the surface. The way to think about this, I believe, is to think of how a lot of photons (light rays if you will) would angle on outside the tank by the time they get down to the surface level if the reflector casts its bounces a little too widely. This has the same effect as if the light had even less wattage as far as the plant leaves are concerned.
Welcome to the first problem set of the world of aquarium plants, lol. With aquarium plants, light is a "skill set," CO2 is a skill set, macro and micro nutrients are a skill set and algae, arghh, well algae is a skill set that requires some mastery of the other 3 skill sets first, yuck! But I exaggerate.. beautiful live plants are well worth the effort of learning how to keep them. Often beginners come in to the tropical fish hobby, pulled by the excitement of the fish but at some point realize that the plants often play an even bigger role in making a tank beautiful or at least share the role.. and the learning can take place over time, its not a reason not to get plants and learn.
~~waterdrop~~
