what are you reading the PH
with? the readout on the controller? a PH
is is the controller opening the silinoid valve when the PH
is reading under 7.0?
if your PH
is 6.5 and the controller isnt letting any more CO2
into the tank..
Maybe your KH
reading will clear things up for us.. If your KH
is under 3 you might be needing to do more water changes to keep the PH
from droping qickly. or buffering the water with baking soda.
Im thinking it might be the KH
or.. your tank got a good dose of CO2
and your controller is just waiting for the PH
to come back up to dose more CO2
again, and its not happening without a water change or some aggitation of the water surfase.
there is a good chance that your controller will be closer to being able to giving a correct PH
reading then a testkit.
just some thoughts...