I'd just check the CO2 good.
Sounds like the rest is okay.

GSA is a PITA and I'm glad I no longer have to deal with it.
Still, it's a two cause algae.
CO2 and PO4.

I think there might be another method also to get rid of it over the longer term also.
But the above does work over a wide range too.


Regards,
Tom Barr