It depends on how high your KH is. The higher your KH, the less effect CO2 will have on your pH. I don't have the link right now but if you search for "measure CO2 pH KH" you should be able to quickly find some of the charts available on the internet. For a given KH, they show you what your pH will be for varying levels of CO2 in ppm.
It's actually, a graph/table that will show you your CO2 based on variations in KH and pH. If you have a small tank, expect a huge drop in pH, like 1-2. You can try adding some baking soda to help out.
This isn't true for CO2, especially if most of your buffer is in the form of cabonate. Cabonate will buffer most acids, but will not buffer carbonic acid (CO2 dissolved in water). As a result, your pH will slowly drop regardless of your kH, unless significant amount of other buffers (such as phosphate) are present as well. But don't worry, carbonic acid is a weak acid and decomposes rapidly in water into CO2 and water.
I don't have the link right now but if you search for "measure CO2 pH KH" you should be able to quickly find some of the charts available on the internet. For a given KH, they show you what your pH will be for varying levels of CO2 in ppm.
