Thanks. A fish person told me to check the CO2 in my pond because
some of my fish were dying, and she thought it might be a good
nonspecific indicator of some kind of toxicity.

Joan

There is a segment on CO2 at http://www.akca.org/ search for CO2 and it will
take you to a page on water quality. It gives no numbers, but describes the
effect on pH and how to control CO2 levels.