Not to be difficult but, dosing nitrates to from 0 to 10ppm didn't seem
to help me at all, although your situation may be different and it did
rule out one possibility. I don't know what the critical level is so I
don't want to go any higher than that 10 ppm for fear of poisioning the
fish. Also, my common pleco doesn't touch the BGA. Other types of
plecos may, I dunno. I haven't heard of any fish that actually eat the
stuff.
I'm not sure if that helped any. It doesn't seem killing BGA is really
a problem. Keeping it from growing again seems to be the challenge.
my aquarium page, info and pics at:
www.geocities.com/spiral_72/Spirals_page.html