I use 2 x 39W T5 HO on my 58g. I don't use CO2 or ferts, but my substrate has 1" of soil in it, so that provides plenty of nutrients for the plants. I have a few different types of swords and they are doing fine. Not growing like weeds, but steadily growing and looking nicely green.
T5 HOs are the most energy efficient aquarium lighting that I know of, providing the most light per watt, so in that sense they are the most evironmentally friendly lights you can buy. They aren't that cheap over here, though.
Damon, do you know of any aquarium lighting that is even more efficient?
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