I think it really depends on your light, fertilizing schedule, CO2, and since we're talking swords, substrate. I can only speak from my experience about these as everyone seems to have different luck with them.
About a year ago I kept Ozelots and Kleiner Bars. Both did very well under 2WPG of Compact Flourescent, while dosing dry ferts, and throwing in some root tabs. Though I have seen swords survive with minimal lighting, no additional fertilization, with only root tabs stuffed into the gravel.