Although normal output flourescent bulbs do lose lighting output over the course of 6-12 months, power compacts from AH supply do not. I use power compacts until they burn out. With nearly 3 wpg PC lighting, you have more than enough light to grow anything. Long internodes are not a sign of poor lighting in this case.
To shorten internodes, try raising your PO4 levels to 2 ppm and keep them there. Keep nitrate ~5-10 ppm and your iron/traces high. Your internodes should shorten nicely, and the Ludwigia inclinata should become much more colorful.
Hope this helps,