[smokinreefer]

it will be easier for you to see the trend, if you take your test a few times a day, for the next couple days, at the same time.
i.e. 10am, 2pm, 8pm for the next couple days.
if the values a renear the same for each time period, then yes, the values are fluctuating due to the fluctuating pH of your input(tank) water.
if the values differ (at the same time frame) something else may be causing the fluctuations.

make sense?