I was referring to cheap hardware store fluorescent bulbs, which are 3000 to 4000K with a CRI of ±80. Not that a good quality 6500K fluorescent is that much more expensive, but they often don't carry those. It would be a waste not to use your LED, it has a good color temperature for one. You could get another LED, if you want a cheap hardware store fluorescent fixture in there you put that one in the back, and you don't notice the color/flatness that much. That's a trick how we did it in the 70's/80's when there just wasn't much choice in bulbs. Yes you can just add the Lumens up.
What kind of algae are they, brown algae? More light means more algae but I don't see algae as a light problem but a water quality/parameter issue; when you have the right conditions for plants they will outcompete algae. I think you said you had almost 0 nitrate that can cause algae as well, you may have to add KNO3 so the plants can take over.
