I probably wouldn't trust a Tetra kit. Go with either Salifert or Seachem. The only other thing that comes to mind, and this is pretty distant, is that the carbon in the Fluval has gotten bioactive and is churning out nitrates. How long has it been in there? I suspect it's the test kit though. Water changes are the only way I know of to actively reduce nitrates.
Life is not measured by the number of breaths we take, but by the moments that take our breath away.