Another explanation that may help you (Tony, and others interested) see the issue here.
Plants need sufficient intensity and spectrum light, and sufficient nutrients in order to grow (= photosynthesize). Duration of the light is basically irrelevant, and it cannot make up for deficiencies in intensity or spectrum. These factors of light and nutrients must be in balance to provide the plants' requirements, and each species can have differing requirements.
Taking the light first, it must have high red, blue and green with sufficient intensity to drive photosynthesis. Once this is achieved, it then needs the 17 nutrients to "feed" that growth. Provided all these requirements are met, the plants will photosynthesize full out, until something is no longer sufficient. At that point, photosynthesis slows and may even cease; it depends just what has become insufficient. Botanically this is called Liebig's Law of Minimum; it determined that plant growth is not determined or subject to the excess of nutrients or light, but rather the minimum of any one of the factors which then slows growth.
Higher plants are more specific in their requirements. Algae is able to manage in almost any situation. So as soon as the plants are no longer using the light and/or nutrients, algae has the advantage and will take it.
The 6500K light is providing the spectrum, no question there. The intensity we do not know, but if the plants were previously doing well (may have been slow growth in your eyes but they were apparently managing well) we can assume the intensity was adequate. However, nutrients may have been insufficient to balance. Fish food and water changes will provide the necessary nutrients, but usually not enough, depending upon the light intensity and the plant species, and the number of fish too. A comprehensive nutrient additive is usually beneficial. I use the word comprehensive for good reason; an excess of some nutrients will be detrimental to some plants--and to fish and bacteria too for that matter. Botanical studies have determined the percentage to each other of the nutrients aquatic plants require, and this percentage is used in products like Seachem's Flourish Comprehensive Supplement and a couple of other similar products. Note well...the level of the nutrients in this product is all that the plants need, subject to knowing how much the plants in the aquarium require, and then providing it.
In a high tech planted tank, with mega light, diffused CO2 and daily nutrient doses, it is less of an issue to overdose "x" or "y." That method compensates with massive water changes to remove the "excess." [I won't get into the fish problems, but there is now evidence that high-tech systems do in fact harm fish.] But in a low-tech or natural planted tank, the accidental or deliberate excess dosing of any nutrient can be deadly. And inevitably algae will take advantage again.
