Have you always had powerheads, or are the ones you just got your first ones? Excess waste and a build up of carbon dioxide can lower your pH. By adding those powerheads (make sure you have at least one pointed towards the surface of the water causing ripples) and possibly doing a partial water change you could start to see an increase in your pH. Even if you can raise your the level to 7.8-8.0 and keep it steady there your tank will be much better off but, 8.1-8.3 are optimal levels.
The most important thing you want to remember especially as a newbie, a steady level in your tank is much better then a constantly fluctuating one. That goes for your pH, temperature, and water chemistry.