do you vacuum your sand bed? I just finish helping a new guy get over this out here. He never vacuumed his sand bed to get rid of the decaying matter. with frequent vacuuming and water changes his tank is now doing fine. It is a Co2 issue you are battling IMO. I would Vacuum your sand bed a few times really good you will be surprised at what you will pull out and go right down to the glass not just the top layer. Basically when you let your sand bed deteriorate it gets compacted and it suffocates, this in turn kills the bacteria in it and makes your tank test results even worse.

Bill