Hm, sorry...I'm out of ideas. Hope you didn't take any offense to my "let someone else try" approach...but sometimes it just takes someone looking at it all to go, "DOH!"
I'd have someone experienced come over, bring a whole box of test kits...look over your tank...and run everything. I have a feeling the answer will pop out quickly...200 nitrates has to have a pretty obvious cause. But, when you stare at something all day every day, the obvious just blends in. (Once again, not saying it's your fault...please don't take it that way...just saying we're missing something in the picture and I can't think of what....maybe someone else will.)