[150EH]

Your pH will be normal in the morning and drop during the day as gas is added, all the fish I have ever had are fine with the pH drop and only bothered by being gassed to death. My kH is much lower than yours and I have never had any problem with the so called limited buffering capacity that I know of, if I were you or me just try to get a good amount of C02 in the tank by watching the plant health and growth along with the reation of the fish. You can test pH and kH and have a drop checker and never really know the amount of gas in the water but these methods can be guides to making adjustment in gas amounts but are rarely spot on acurate. You don't want to add so much C02 that your pH drops so low that your nitrifying bacteria slows to a point little activity and by adding oxygen to the water column with air stones/wands during the lights/C02 off period you can help build a larger base of nitrifying bateria which will help the tank as a whole perform better, so example lights/C02 on 8 hours, air on 16 hours, can equal good things, good luck.

Don't forget about circulation to distribute C02, nutients, etc. equally around the tank.