alrighty ill take a shot at this one based on past situations but im not sure if my two ideas are 100% correct.
But if i had to take a shot at it, i would say its either
a) your tap water ph reading was 5-6.0 when there was a large amount of chlorine in the water. but after the chlorine dissolved the pH spiked for this reason.
or
b) water is a really efficient chemical. maybe the pH lvl was checked at first when water was put in, but over the 12 hour period the water went through a simple Diffusion process. diffusion is the movement of particles from an area of high concentration equalize to an area with lower concentration in order to balance itself and reach equilibrium. therefore your initial pH measurement could have been bogus.
if i were you, i would try to keep a nice growth chart
and write down your readings and temperatures and other stuff and over a series of time you could solve your problem with consistencys
holla back whoooop whoooo
