firstly you can not have a kH of zero, by the scale the lowest kH is 1.
secondly if your established tank is running at 250ppm for no3, there would be no living animal in there.
I suggest that you invest in a decent test kit, like API freshwater master kit (around £15) as this uses liqued chemicals rather than a litmus strip, which are very inaccurate.
don't worry about yor pH, a level of 6.4 is ideal for most fish.
It is far better to have a stable pH than one that is continually fluctuating