Ivan, like what my chart shows, even when the MH hrs is extended by 1 hrs to 7hrs the pH just raise and don't seems to be tapering off.
I see the pH begin to drop sharply after the main lights goes off.
Have tried to run the MH for additional 3 hrs from 9pm to 12midnight and the pH trend shows that it is raising too. Interesting isn't it?
As I don't run my CR 24/7 but only 6hrs/day now, my kH varies between 6.7 to 7.4. The lowest point 6.7 around 4pm before the CR starts running and the highest point around 12noon when the MH comes on.