You have to test correctly or the readings will be inaccurate. 10 drops of bottle #1...shake like crazy for a 30 seconds ( or is it a minute?). 10 drops of bottle #2 ( bottle needs to have been shaken well prior) and shake for a minute...wait 5 minutes and check out color.
High nitrates are good at the end of the cycle. Were they 160 after the change? If so, do another and probably closer to 50%.
I kinda look at it like chocolate milk...if you take a cup of that and remove 50% and add 50% white milk, you have lighter milk, but there is still chocolate. You want as little chocolate (nitrates) as possible. There can't be too much that it would not come out if new is not being added.