I concur with geomancer, but will expand a bit.
First question is, what sort of plants do you want to grow? Plants need light, but there are roughly three levels in this, what we term low light plants, moderate light and high light. The fixture you now have is only going to cover low light and some moderate light plants.
Fluorescent tubes come in various lengths to fit the fixture, and all tubes in a certain length will be the same wattage, basically. Some manufacturers now make more energy-efficient tubes, so the wattage is lower. Watts is only the measurement of the amount of energy it takes for the tube to light. The intensity of the light it produces is determined by the phosphors inside the tube, and these also determine the colour (measured in Kelvin). Most 18-inch tubes will be roughly the same in light, depending upon the type. Some are better than others and produce more intense light.
So you have two options. Staying with the present fixture, buy two good tubes to provide the best light possible with this arrangement. Or, replace the present fixture with one that takes 48-inch tubes.
Staying with the present fixture, I would recommend two tubes. The Hagen Life-Glo is one, and the ZooMed Ultra Sun is the other. These both have a Kelvin of 6700K (the ZooMed might be 6500K, much the same) and provide good clean light. With these you can have low light plants (Anubias
, crypts, Java Fern
, Java Moss
) and some moderate light plants like green-leaf swords, Pygmy chain sword
, Brazilian Pennywort
, and floating plants like Water Sprite
Hope this helps.