PH too low.

I have had a 35 gallon tank now for about 2 months. The water is super
clear and the fish seem to be happy and well adjusted. My question is how
do I get my ph above 7. I have tried a ton of those tablets that fizz and
and powder that supposedly will fix it but it still remains very low like
even lighter than the chart shows on the dip stick.

Also how often should I change my filters I have 2 that go in the box. I
think it is a aqua tech 150 or something like that.

How often should I replace the water and how much and what kind of water
should I use? I have been using drinking water or spring water but it is
very hard.

Jason