Sworrall,

Why are you injecting CO2 at night? You should be injecting CO2 during
lighted hours in order to facilitate photosynthesis. At night, plants
go into respiration in which they use oxygen instead of CO2. When you
inject CO2 into the tank, your PH will drop because CO2 creates
carbonic acid, which will bring down your PH. If you do not want your
PH to drop, you may wish to add a buffering agent, or you may wish to
do more frequent water changes, though based on your GH and KH, water
changes aren't really going to help you much.

....as I read your post again, it sounds like the problem you are having
is that as soon as the lights go on, your PH reading changes and this
also occurs as soon as you turn the lights off. If this correct? If it
is, I would hazard that your PH meter is faulty.

Hope something that I said helps, but I'm no expert so maybe someone
else can offer better advice.