in case of hard water, improve the CO2.
Hehe, now you sound like me.
I had GDA with both soft tap, and Hard KH's. 11 and then 17 degrees.
KH here in Sacramento is a mere 18-20 ppm, pretty soft, soft as anyone ADA or other tanks. I do not add CO3's. It'll go up to about 35-39 ppm during some parts of the fall, otherwise, it's VERY soft.
I dose 30 ppm a week of NO3 as KNO3, 10 ppm or so of the PO4, lots of traces and about 1 table spoon per 60 Gal of tank of GH booster, but my GH is about 2 degrees in the tap.
Without a control for a test, eg a tank that is algae free, you cannot test dosing or nutrients independent of other factors for algae causes/cures really. If you are convinced of something, you then need to go back and induce the GDA and then apply the treatments you think will best logically support your hypothesis.
I cannot with GDA, because I have trouble inducing it. I've managed a few times, but.........So all I can really do is falsify. This is much more helpful than many assume(falsification).
If you can grow and induce GDA easily, and consistently, then you can master the growth of the alga. Then you can do test upon it. This is the key to understanding algae.
Same with plants, if you can grow them really well, then you can understand their growth really well. If you cannot grow one or either, well........hard to say. No way around this issue.
GDA is present in hard and soft KH waters. I think there is some truth to it being more common and nagging issue in low KH tap waters, but clearly, my tanks fall into the very soft grouping and I have no issues with it.
But.......this might help a few folks by adding more KH if they are in the low KH range. ADA adds KH, but not much.