http://dataguru.org/misc/aquarium/calKH.asp
The above webpage contains a very useful tool for making kH and pH adjustments.
First off, Id do water changes of 25% for 3 consecutive days to get the nitrates down a bit. I'd also add a TBSP of Marine Salt for every 20 gallons of water to raise the pH a bit, and re-take the measurement. Before you do that make sure the kH is at 3 or higher, if not add the baking soda solution per my previous post on pH and nitrates, or per the link above (Remember, raising kH will also raise pH a bit). Make any adjustments small and gradual, and wait a few hours between adjustments and measurements.
Never assume the new water you add to a tank is at 0 ppm nitrates, so test the water you are adding for nitrates. My bet is that its already at 5-10 ppm nitrates.