been doing some thinking and looking and im putting 2 and 2 together to come up with this.
could your gha be feeding off the nitrates in your tank? and maybe your dsb
has less to do with your 0 to near 0 reading of nitrates? and at one point in time wasnt pH a concern? is it still? if so have you done what mr_X has suggested with pointing the power heads at the surface for better gas exchange?
sorry if i missed the parts where you got all this figured out... also i dont remember if your friend is in town yet but if so, hope you guys are having a good time.