I've had Cardinals for a week now, they seem to be doing fine. The PH level is 7.6 I read that the PH should be between 4.0 and 6.0 Should I be concerned?
7.6 is a little high, but shouldn't cause any long term problems. 4-6 is required for breeding, but neutral to slightly alkaline is fine for day to day. Clean water is more important. If you would like to lower your pH a bit add some driftwood to your aquarium.
Not long, under a month. I knew the pH they are supposed to be kept at before I got them but after reading about plenty of people keeping them in higher pH I decided to risk it.
Stable pH is far more important than correct pH. More than likely if you got them locally, they were already used to your local tap water's pH. no need to try and match their native conditions unless you were going to try and breed them. Far too much emphasis is placed on pH. Water quality and stability is much more important. pH is simply how acidic or alkaline the water is. A rapid change in hardness however can and does harm fish (resulting in a change in the pH value and often incorrectly referred to as pH shock.)