What are you dosing? GSA is usually a sign of low PO4, though noon bursting you lights wont hurt either
I have a similar problem but my phosphate is 6ppm out of the tap according to the City of Dayton. I still get green spot if I don't dose enough PO4 so I would assume whatever PO4 is in our taps isn't available to the plants. Try adding 1-2 ppm of PO4 either by KH2PO4 or Fleet Enema and I'll bet your GSA won't return once you clean if off of the glassRight now my Co2 is 2 hagens reactors and excel, I dose Fe3+,K , N, Trace and seachems flourish, I havnt been dosing P04 because my tap water has 3ppm already.