Well, my take on the whole light situation is from a step backwards... There are two things at play with the light thing - first, the amount of luminous watts are actually captured and used by the algae (Intensity of the light, duration that it's on, and the area over which the intensity is captured.) The other thing is the intensity of light that the algae can actually use. So putting a 1000 lumen light, concentrated on 1 square inch, may burn the algae (high intensity), but spreading it out over 100 square inches may be perfect (Moderate intensity), 1000 square inches is probably not enough (low intensity).
Using ammonia and nitrate byproducts as food is an endothermic process, so it requires energy input to happen. As such the rate that the nitrate can be removed is directly affected by the rate at which energy is put into the system (lumens).
So, you need to get two things right. Most important is the intensity. It seems that FW may require lower intensity than SW, so a larger area may be needed. The second thing is that with a given intensity, your light and screen combination must be large enough to capture and use the light energy at a sufficient rate to counter the rate at which Nitrogen products are being produced in the tank. This is easier to get right, as you can just build too big, and the algae will use whatever it can. Even if you go too small, the scrubber will help, it just won't do the whole job.
