I say keep it at 5.6 to 6.8. Plants eat more nitrogen at 5.7 to 5.9 that at 6.7 to 6.9. But they consume more iron and magnesium at 6.5 to 6.8. So you want a fluctuating PH to get all the nutrients. I have mine at 6.0 to 6.5. Since the nutrients breaking down in the soil is going to lower the ph and you want it to fluctuate, I put it in a little high. pH is adjusted with acid and alkaline solutions normally called pH up and pH down. You can get these at pet shops and hydroponic stores allover.
