With a KH of 4, your PH sounds right for a non CO2 enriched tank. Any chance you tested the PH shortly after a water change when your readings were lower? Tap water is compressed so it contains a lot of CO2 which will bring down the PH. Once the CO2 has gassed out, the water will stabalize to atmospheric concentrations of CO2, bring the PH back up. Wait at least 24 hours after doing a water change before testing the water. A few swings here and there in a new setup are not uncommon however, trust your current test results, they sound right on, you'd need to inject CO2 to get your PH down to 6.8 with your current KH.
Regarding the plants, if their needs are met, there is little preference in PH alone. Plants like CO2 (among other things), CO2 brings down the PH, perhaps this is why many say that plants like a low PH, but it's really just a secondary effect of the higher CO2 levels, that's what the plants really like.
Hope that helps
Giancarlo Podio
Regarding the plants, if their needs are met, there is little preference in PH alone. Plants like CO2 (among other things), CO2 brings down the PH, perhaps this is why many say that plants like a low PH, but it's really just a secondary effect of the higher CO2 levels, that's what the plants really like.
Hope that helps
Giancarlo Podio