What you need to take care over is that when you buy a tube which is given a certain K rating all the light generated will not be as stated. There will be peaks and these peaks are what realy matters. Some people )eg the Dutch and the marahoochy growers have done a lot of research on what K ratings do what. We have shops here where when you enquire about what type of light is best you get an answer that would be very useful to a person growing da erb---this will grow plants wide and short and this will encourage budding. I have played with some of this info while growing Echinodorus sp emersed and have found that any changes in growing conditions (temperature, type of light and photo period) are about as usefull in encouraging their desire to propagate. I have had many conversations with Bob Ward from Redwood Aquatics who is very experienced at growing plants and has travelled the world to see the conditions many of these plants are growing in when in the wild. He always goes back to the good old days (he is another oldie) When we used to grow these plants successfully with ordinary light bulbs. The type of light in my opinion is frequently over rated but if you feel it is important you need to look at where the peaks are on the tubes you are looking for.
In a previous life, when I first started growng plants there was no emersed plants in the shops and I was supplying much of the south Island with plants. The people regarded as the experts in growing plants were the Germans and they used a growlux, a cool white and a tenth of the wattage in incandescent. That is what Bob and I used when we graduated from incandescents. Worked for us both and is a lot cheaper than many lights being pushed at the moment.