I don't know why anyone hasn't asked what nutrients you're using. We have 5 outdoor basins. Three are emmersed set ups with no circulation, about an inch of water, super high light, nutrient rich substrate and water, and of course beautiful plants. Two are deep water basins. One with a few pumps and one without.
When nutrient levels drop in any of these, algae strikes.
Now, keep in mind that ALL of the basins get full afternoon sun for at least 6hrs. Mind you, we're in Texas so if you want to talk about intense.
So, I ask, what nutrients is it getting?