Hatachimog, reading through your post it appears that you have been having a very similar issue to one that I had about a year ago. I went through a similar set of circumstances and was about to pull out my hair. Talking with my LFS, and with no other options, we decided to test the gravel. I pulled out about a cup worth of gravel from the bottom of the tank, thoroughly washed it in warm water. I then put it in a small dish and added a bottle of water (used an actual bottle so that I knew it was clean). I then tested all levels immediately to confirm that they were zero. After this, I put the dish up on a shelf and left it alone. No fish, no food, no ornaments, etc. Three days later I tested the levels and found that they had spiked. I swapped the gravel out and started the cycling process fresh, but since my tank has been great. I saw your other post about the Prime. I used it and do like the product. However, before you keep throwing money at your tank, I would suggest that you try what I did. The worst thing that happens is that you rule out one more thing. Hope that it helps and I feel your pain.