This calculator allows you to make a reference KH solution for a CO2 drop checker using some container, NaHCO3 (baking soda), Bromothymol blue (such as from a low-range AP pH test kit), and any target CO2 ppm. Keep in mind that this target should be the perfect green on your drop checker (6.6 pH). This calculator assumes humans can see "green" at +/- 0.2pH.

So, if we use that assumption with two drop checkers, we can calculate CO2 anywhere from +/- 2.5ppm to 30ppm.

pat_w provides another example.

image courtesy of pat_w

Another way to do this is Hoppy's method of targeting the color yellow, which is much easier for some to read.

image courtesy of hoppycalif

This project is heavily influenced by fellow aquarists and particularly Hoppy, who you'll find posting on plantedtank.net and barrreport.com

Drop checker example images, in order, were taken by wet, patw, and Hoppy, who are active on the above forums as well as APC

The pH chart is courtesy of Aquarium Pharmaceuticals and is available at most any local pet store.

This calculator and page was built and is supported by wet and is released under The Whiskey License. You can do whatever you want with it, there is no warranty, but if you like it and want to say 'thanks,' be aware I like tasty whiskey! If you find bugs, please let me know!

The source for this calculator is on GitHub and this is version 0.2.

Version 0.1 is still online here.