single bulb, 18w?
I think it will be borderline with what you currently have. Unfortunately, with florescent bulbs you cannot just buy higher wattage bulbs, like you can with incandescents. The fixture is made to except 18W bulbs.
The 5000K is a little low, but is close enough imo.
If it is single bulb, plants will be a challenge... but java moss, java fern, anubias, they might stay alive under that light (though they won't grow much).