I've recently solved my green spot algae problem by raising my lights and dropping from 10 hours to 8. But I was wondering if light is the main factor in GSA then why does it seem to affect the bottom leaves and lower part of the glass? I'm not just talking fast growing stems where new growth over takes older gsa riddled leaves making it look as if the bottom got it worse. The bottom leaves of my crypts would have lots of gsa while being covered by large healthy upper leaves. And the bottom portion of my glass would be loaded while a few inches from the light would clear.
I noticed zero GSA above the water change line back when I had GSA. It doesn't like being dried out. Just now I noticed my external overflow box's u tube that refuses to stay full hasn't any GSA while the full u tube has lots of GSA.
Perhaps the new leaves are more vigorous and able to shake off the algal spores?
When I increased the amount of phosphate dosed there ceased to be any GSA in the tank proper. I wonder if my snails and otos can graze it better now in the tank and cannot get to the u tube to clean it off?
What is your phosphate level? GSA is the result of insufficient phosphate level + high light. I will get GSA when my phosphate level is below 5ppm. The first area I would see them is near the water surface, near the light source. I think 8 hours of light is enough. You may not need to elevate your light at all if you have sufficient level of phosphate.
Maybe there isn't enough CO2 in the water as well?
What algae eaters do you have? I don't think it is my brilliant tank managing that is keeping algae away, I really think if you have it right it helps but but algae eaters do a lot of work as well. After the time a small ramshorn returned to a tiny sword plant for a couple days in a row and completely removed GSA that completely covered one leaf I know they are doing a lot for the tank. That tank had a monster case of GW at the time and after the water cleared I saw no GSA. Whether the GW created a situation where the GSA softened or there wasn't any of the other algae growing due to the GW I don't know.
I dose half the phosphate as the nitrate and haven't any idea how much that is, tops the phosphate chart. I could dilute tank with my zero phosphate tap water to see how much I have but haven't yet. Plants look better as well, a more vibrant color.