All I know is BGA turns off because of the high nitrates.
Then dump in a lot of nitrate. You know exactly how much you're adding. If this turns BGA off, then it no longer depletes nitrate. In a high-light, heavily planted tank, plants typically use about 1-2ppm daily; if you have fish, food can easily add that back in and more. You're assured to reach at least your goal, no test kits necessary at all. If what you say is true, then this should
be a surefire and foolproof cure, 100% of the time.
Look beyond your personal experiences to the many reported on this forum, and you will see this is clearly not the case.
This coming from someone who has had multiple BGA infestations, in tanks with heavy fish loads and EI dosing. Though I don't know my exact nitrate levels at the time of each infestation, I can say with certainty they were always excessively high, certainly high enough to "turn off" BGA immediately if that were possible. Yet it exploded each time I introduced an infected plant, effectively treatable only with antibiotics. It was only later, when both my skills and overall plant health improved, that this no longer happened.
I have BGA now. It's growing happily on several HOB filter outflows. While the outflow apparently provides a suitable environment, the tanks themselves do not, as it does not spread from that location. I don't know exactly why, but it's not due to nitrate levels, as they're up to 70ppm.
I'm not trying to start anything either, not a bit of malice in my reply. But really now:
"I don't think it's BGA though if indeed you kept your nitrates that high for a week or two."
The pic showed it was BGA.
"Yep, BGA all right. You have low nitrates."
His nitrates were fine.
"I don't think the test is good. BGA is better"
Admit it. You're stretching for an explanation that fits your understanding of BGA at this point.
And if the OP gets another test kit and gets the same result - which I believe it will - what is the next explanation? That test kit is bad too? Something in the tank is interfering with the tests?
Been there, done that; and I believe the end result will be to leave the OP more frustrated, with the problem still unsolved, and needlessly distrustful of test kits. We may disagree, but I do not want him to end up nuking his tank. They are lovely crypts. That's all, really.