My bazooka is back hanging over my display.
My lfs recommended vibrant so I have just dosed for the second time (my nutrients were already reading zero before starting this)
I don't know how Vibrant works, or even if it will work but I am finding GHA actually more frustrating to deal with than dinos were.
After a good look around the back of the display there is a lot more algae than I thought so maybe it is using up all the nutrients before the test kits can measure it.
I followed a Vibrant thread for quite a while. This bacteria blend seems to do a lot of good things against algae when used properly. Like most bacteria supplements, it does lower Nitrate so keep an eye out there. Perhaps it will counterbalance that consumption by killing off the algae that is presently consuming your nitrate.