Platy behavior - ph question

[wertyu5555]

Hello there! i have two questions and i was wondering if anyone can help me.
First, i have noticed for the past few days a bizarre behavior in my platys.
The one (i suppose the female) is sitting in a certain corner of the aquarium totally still and the other one is always over it doing some mechanical movements. other than that they both eat normally and they look very well. What could be going on with them?
My second question concerns ph. the ph of the water i'm putting in the tank is 6.8 but when i measure it after a few days its 7.6. what is messing with my ph?
i would appreciate it if anyone could help me with this. Thanks

 

[clutterydrawer]

what decor do you have in your tank? any rocks? where did you get them from? what kind of substrate are you using, and where did that come from?

 

[wertyu5555]

THe decor of my tank is a few plastic plants and lots of hiding places , and the substrate is small pebbles. I have these platys for two months now