Go to your local petstore and get bacteria culture and a few of the fish you are wanting and put them in. It is not the ph you are wanting to fix. What you are needing to do is cycle the tank for a few weeks get a test kit and study aquarium cycling what you are looking at is ammonia nitrites and nitrates. I study fish for living if you have any questions I will be more than happy to help!
If you are just starting the tank get some cheap gold fish and let them swim around for a week or two in order to get the water ph levels in line. Then go get your " real" fish. Set the bag in for a few hours so the water temps equal out. ..... Then let them free!