pH Levels.

Hi Folks,
I'm a newbie to the pastime, so please, bear with me.

Setup:

Jewel 120L tank. (28G?)
Standard Filter. particulate filter cleaned/replaced every week. Others
as per recommended schedule (Activated filter changed once, others
untouched.)
Standard Lights - On for 5-6 hours a day.
2 Inches of gravel, washed and boiled.
Well-ish planted. Some grasses and a couple of Amazon Swords.
6 Inch Airstone, on for ~90 minutes a day.
Temperature - 78f.
Hard water area, pH from tap 7.1 - 7.2

Tank ran for 35 days to cycle it, with no fish.
NH3 - Nil.
NO2 - Nil.
NO3 - 5 ppm.

Tank is not in direct sunlight, but in a south facing room.
I have had 6 Zebra Danios for 3 days, tank is now in day 38.


Issue:

My tank pH seems to settle at about 7.5 - 7.7. I have treated it
previously with a pH down (Aquafin) solution - making it 7.2 or so, but
the next day, it will climb again. I don't think this is the end of the
world, but in the long run I want this to be a community tank, Danios,
Guppies, Tetras and a bottom feeder or two. I have been told that a pH
of in or around 7.0 is desirable for this.

I thought it was the Airstone (I had it on for over 12 Hours a day, but
have dropped it's use considerably).

Any idea why my pH is doing this, or am I just being a Donkey?

Any help at all appreciated.
Rob.