Alkalinity-CO2-Test Kits

We have set up several planted tanks w\CO2 and TWP DI water
(reconstituted with their Electro-Right @ 8 tsp per 20 gallons and
bicarbonate of soda @ 1.25 tsp per 20 gallons). We came up with KH 3, GH
2.5 (Tetra Hardness kit). We were shooting for PH 6.7 which would give
us a CO2 concentration of 18 mg\l according to George Booth's graph.

In an attempt at greater accuracy we purchased an Oakton PH meter (w\.01
resolution and temperature compensation), Hach Total Alkalinity, Total
Hardness, CO2 and PH 6.5 to 8.5 Phenol Red. Now, instead of greater
accuracy, we have acheived total confusion :-).

Anyone that can help shed some light on our befuddled state would be
greatly appreciated. Total alkalinity measures 68 mg\l (4 dkh). Total
hardness 60 mg\l (3.5 dkh), which is 40 mg\l calcium hardness and 20
mg\l magnessium hardness. Which number should we use to stay on the
PH\KH\CO2 graph? The CO2 test kit has a very indestinct end point as it
is such a light pink. I called tech support and they said go for the PH
8.3 endpoint on the meter which is not pink at all! I've calibrated the
meter very carefully and it will read my buffer solutions 4.0, 7.0, 10.0
precisely but the Hach PH kit reads off the scale low when my meter
reads 6.85. The CO2 kit reads from 12.5 ppm (meter 8.3 endpoint) to
21.25 ppm visual extremely light pink. We don't know what to believe and
don't want to kill our fish, although our plants are doing great! We
want to keep the water as soft as possible and still have room to inject
enough CO2 to maintain 20 - 25 ppm and to maintain a PH range 6.6 to
6.7. Chocolate gouramis are the intended inhabitants of our 90g planted
tank if we can stablize conditions and be confident of our testing
procedures as they are rather finicky as to what they will spawn in.
We've had some success in the past and hope that a heavily planted tank
will be just what they want to complete their breeding.

Thanks for your time,
Daniel and Rebecca Marsh
In currently sunny Gold Bar, Washington where the tap water content
changes almost as fast as the weather.