It's due to a lack of CO2. It's not so much that you aren't injecting enough, it's because it's not circulating properly. I've been carefully observing this for the past year, and every time I get a little GSA on leaves, it always shows up when circulation diminishes. The higher the lighting, the tighter the tolerances. Getting the nutrients right is relatively easy, but the proper use of CO2 is far more difficult. Focus on keeping plants pruned regularly so as not to obstruct flow and be sure there is adequate, even flow throughout every part of the tank. It's not necessary to run high light if your CO2 can hit on all cylinders. It may take longer for plants to grow, but they will grow with exceptional health and coloration. It will also give you more time to observe and make adjustments.