I don't think you will have a lot of luck with just one tube. However
hornwart, being a floating plant and therefore closest to the light,
is probably suffering from something else rather than insufficient
lighting.

2x40W T12 tubes or 2x32W T8 tubes work well for low light setups, I
think this would be the minimum you want to consider. If you have CO2
on it's way you may want to consider a little more light. The
all-glass tripple T8 fixture worked very well in my 55g tank, as did
2x55W compact flourescent retrofit from www.ahsupply.com

If you can reach around 2 watts per gallon you should be able to grow
just about anything.

Hope that helps
Giancarlo Podio