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Ph Drifting Down Between Water Changes...


Over the last month or so I have found my average PH to be slowly drifting
down. It used to be around 6.9 now it is 6.65. Here are some details.

Tap- ph 8.2, kh 4, gh 8
Tank- ph 6.65, kh 2, gh 8 (before lights this AM)(changed water yesterday )

I have Co2 at about 2 bubbles /minute in a diffuser in my sump. I use
Fluorite gravel and no calcium containing rocks. At water change I add TMG,
KCL, MgSO4, Seachem Iron. Nitrates are more than 1, less than 10. I have
two big pieces of driftwood which after 6 months are finally slowing
release of tanin so that I greatfully only change water once a week
now(40%). I also add a bit of barley straw extract (2cc/week). Plant growth
is good and light algae growth is on a decline. Plants look good and some
have flowered. I use a PinPoint ph monitor which calibrates easily and has
a newish probe. I have pearling in the tank but it is only pronounced right
after a water change...

What is my question? I am just wondering why the stable ph of the tank has
dropped from around 6.9 to 6.65 over time...  Should I just call it a day
and enjoy the tank or am I heading for a problem. If I look at the CO2
chart I am at about 15mg/l for CO2 which seems fine. 

Any thoughts are greatly appreciated...

Thanks a lot,


           Paul Mouritsen.....mourip @ erols.com