I had never heard of a direct CO2 test kid. I always have done it the way
you mentioned in the second part of your note.

Bob
"Dave Millman" wrote in message
...
James wrote:

This is my first time testing for CO2 in my fresh water tank.
its a 180Gallon tank with a few tetras in it. The Salifert test kit
shows that I had to use 15 drops before the test water turned blue.
From what I read 1 drop = 5mg/L or PPM CO2. Was my test water suppose
to change to blue after 4 or 5 drops rather then 15?

There was a CO2 test kit in the master kit I purchased way back when.
After much reading, I learned that it is significantly easier and perhaps
more accurate to calculate CO2 concentration from pH and KH, for which
the test kits are easy and relatively accurate. This works as long as you
do not use buffer chemicals for pH adjustment. KH adjustment using Sodium
Bicarbonate or Calcium Carbonate does not cause a problem.

Read about it he

http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm