Probably nothing. Once the aquarium is running with fish for a time, the biological system will establish itself. We know the GH is 8 dH which is moderately soft/moderately hard, but we don't know the buffering capacity of the water (the KH or Alkalinity). I myself would expect this pH to lower over time.
My tap water has zero GH/KH and the pH is 7 (they add soda ash to raise pH to prevent pipe corrosion). In my aquaria, the pH is well below 7, depending upon the individual tank; in some it remains around 6.5, in others it is in the 5's, in others it is below 5 (can't measure that low). You have more buffering capacity than I do, presumably, so it will have to sort itself out.
The only way to lower pH now, initially, is to dilute the existing GH/KH with pure water. That is not as easy as it sounds, so I would suggest leaving well enough alone and seeing what transpires. Fluctuating pH is much more detrimental for fish than a stable pH even if outside their preference.
