I'd still focus more on CO2, if the PO4 is up as well, you should not have GSA issues.

If you get GSA or any algae after a trim etc, do a large water change right after the trim or any work inside the tank as a rule.

also, when you change the water, make sure the drop checker is changed at least once every week or two, keep up on that also and realize..it's got a 2-4 hour delay in color resolution.

So some tweaking with your eyes and watching the plants,, should help.

Regards,
Tom Barr