Well put Warren,
to further add to it, I have found that by putting a two foot pink tube ( red and blue spectrum) as well as two three foot white tubes( std bulb and fitting straight outta an office block) I have managed to avoid excess algae, I was gtting overrun with it and was loathe to use an chemical algae remover unless absolutely nescacery(sp).
I originally had just the two foot pink tube on my 3 foot tank, I had always intended on putting in extra lighting and did so. When I added the white lights the algae was growing quite strongly, but after adding the 2 extra three foot whites it seemed to slow down and then the algae started to die off.
Don't know if the extra light or dilution of the red and blue had any effect. but there are only a few spots of algae i the tank and I haven't put a drop of any chemicals into it other than the iron and c02. The plants are still growing strongly and regular priunings are now required.
Hope this helps
B