Hmmm... I'll throw some thoughts your way...
Is there any reason why you keep your tank at 80 degrees?
Your lighting isn't high, and I see you are not dosing N, P, and K... this should be fine. Is there any chance your nitrate is coming from your fert tabs? I'm not sure what is in them... do you disturb your substrate a lot?
Have you calibrated your test kit? Tested your tap water?
I agree with the others who have suggested one large weekly water change is likely to be more beneficial to several smaller ones throughout the week (not meaning to belabor the point)...
I agree with racer... clean off as much as you can, then do large WC, and make sure your filter is clean (of course you know to preserve your biofilter... I'm just including that for others).