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Re: Aquatic Plants Digest V3 #962

I'm writing because I think that this story might be helpulto someone.
I have 125G tank with pH controlled auto(Neptune) CO2 injecion.  The
tank had been stable and healthy with plants and fish until about 2
months ago.  I noticed increased CO2 consumption and stress in the
fish.( breathing at the surface and an occasional death).  I
recalibrated my pH Electrode in a packet of standardized 7.00 solution
and it checked out perfectly.  Pretty soon my tank pH reading was
creeping up and the CO2 was being injected constantly.in spite of this I
could not keep my pH below 7.8! 
The fish were doing worse.  Recalibration of the pH probe revealed no
problem.  Finally I decided to ake some tank water, put it into a glass
and put in the pH probe and measure the pH.  The water that had measured
pH 7.8 in the tank now measured 5.8 in a seperate container! Oddly I
deidetopu the ORP probe into the gass with the pH probe and the pH value
returned to 7.8.  When I disconnected the ORPprobe and returned the pH
probe to my tank iread 5.8.
Apparently there was some interfence from the ORP probe which caused
inaccurate pH readings and with my moderately hard municipal water I was
poisoning my fish with CO2 in the course of maintaining  a pH of what
was really 5.8. 
My calibration method missed the ORP interference.  I learned some
things and I'm sure ther are other lessons as well:
1. Use a pH kit to check tank water
2. Don't ignore stress in fish inspite of good electronic measurements.
3. Don't ignore increased CO2 consumption( I told mself that I got some
flat cylinders or that I had a leak.
4. Although I never really used the ORP value, it was a potential source
of system error
5. CO2 in excess is toxic

Having simply removed the ORP probe, my pH has returned to 7.00 and is
easily maintained with very little CO2 injection time. The fish are fine
and the plants are doing great.