Richard Sexton wrote:

You're not dosing with Potassium nitrate. That's your major problem.

And fits __precisely__ with what I've said for several years about BGA.
Low NO3 causes BGA spores to germinate.

He's adding PO4 from the well water, plenty of traces, some fish waste
etc, everything except extra K and NO3.

Folks that try and water change the BGA are often very frustrated
because once it's there, it's very tough to get rid of and can live on
very low nutrients, like Green water...........green dust algae
zoospores etc

You need to provide a nice range of nutrients for plants in order for
them to grow well, not too low= algae, not too high= dead fish from
very high NO3(100ppm or more using KNO3)

EI is a simple method to do that so you spend more time feeding fish,
gardening and less time fiddling with test kits, calibration solutions
etc....but some folk enjoy that....

I think there is a water testing forum somewhere, that's "their" hobby.
haha

Regards,
Tom Barr