you can also measure O2, and know that O2 is consumed which increases CO2

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).

IF Kh is unchanged and PH is changed then CO2 has changed,

True, but as per my previous post KH is changing in my filter

did I assume kinda, but only
kinda this is how a coil works, yours i have no idea, i can kinda guess
from what your
saying, and maybee something is consumed the sulphur maybee?? along with
Kh, or maybee the
sulphur alone will lower Kh

Not sure but dropping Ph seems to be a generic issue with de-nitrators - I
wonder in fact if that is a mis-nomer & it is a secondary effect of dropping
Kh. If you have a coil running now I'd be interested to know if there is a
variation between the Kh in your tank & that coming out the coil.

rgds

I.