OK, time for a silly question..... why do you want to bring your PH lower?
I mean, at 7KH and 7PH you have 20ppm of CO2, that's a good level IMO. Your plants are even pearling, why do you want to fix something that is not broken?
If you are targetting lower PH levels because of certain fish, then lowering your KH will be better, but it's not going to make much of a difference to your plants IMO. Otherwise try to reduce surface agitation to reduce the amount of CO2 that is lost. Still, I'd be happy with your levels, plus at 7KH, PH changes due to variations in CO2 levels are slight, often too little for our cheap PH test kits to give us an accurate reading. At PH6.8 you are borderline with what is considered too much CO2.
Just my two cents worth