Check out this article, if you haven't read it:

http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm

Among other things, it talks about what normal levels of CO2 are, and things
that can throw off CO2 readings.

I use pure R/O DI water, remineralised with Kent R/O Right and pH
stable.

Try leaving out the pH Stable. I believe that's just baking soda. That will
give you a sharp bounce in pH, but it doesn't stay.

What size is your tank, and how much light do you have over it? Also, how are
you adding CO2? (DIY, compressed, some other method?)

Often, if the pH is higher than it should be (based on KH), it means the plants
are stripping all the CO2 out of the water, artifically raising the pH. The
solution is to increase the amount of CO2 you're injecting, or lower the
lighting level.


Leigh

http://www.fortunecity.com/lavender/halloween/881/