I cant totally help you only guess, I KNOW that the CO2 in the coil
denitrifer is
increased and O2 is decreased and Kh is un-effected IF the one you have
doesnt do the same
thing, then thats different. in a coil system bacteria consume oxygen
while its in the
coil before it reaches the bio balls, where other bacteria consume nitrate
and something
else (comeon chemistry experts tell us how its done )

Same bacteria, my filter is just stuffed with pure Sulphur (supposedly to
feed the bacteria). Interestingly enough the cannister is clear and the
instructions make no mention of keeping it in the dark - in fact the only
thing they say is that it must be upright - they suggest you mount it on a
wall! Mine is in fact mounted inside the cabinet under my tank so it is
basically dark in there.

How did you test for the increased CO2? If it is happening like that I
wonder where the carbon is coming from?

If you have assumed that the CO2 is up because the Ph is down then I'm not
sure that's what's going on. I tested the Kh of the water coming out my
de-nitrator and it is markedly lower than the Kh of the water in the tank.
That would have a side effect of lowering Ph as well (without needing an
increased CO2 level to do it).

rgds

Iain