pH at 8.2 question
So I have a 20 gallon long that has a pH of 8.2 (actually someplace between 8.0 and 8.2, hard to read the exact color on the API test). I'm trying to raise some Cardina shrimp, so ideally the pH would be closer to 7.0. All the other parameters, listed below, are good. It is a heavily planted tank, about one month old. All the tests are run using API bottle tests.
Any idea why the pH would be where it's at?
Source Water - Rain water with Salty Shrimp mineralization. The pH of the rain water is 6.5, everything else is 0
Ammonia - 0
Nitrate / Nitrite - 0
Phosphate - 0
Gh - 6
kH - 5
Substrate is black diamond blasting sand
One airpump gently rippling the top (not too much aeration)
No driftwood or other stones
There have been a few chili rasbora in the tank as it cycled but no major bioload
Lighting is Twinstar 650 on a 6 hour time cycle
When I set the tank up a month ago, the pH was around 7.2. I didn't pay much attention to the pH the first two weeks and then was surprised to find it at the current level.
The plants are all healthy, no dying leaves. The fish are fed sparesly every other day.
Any ideas? Does Black Diamond affect pH ?