Measuring the saturation of CO2 in your Aquarium

In planted aquariums aquarists often utilise CO2 as a means of keeping plants healthy. As such it is important to keep an eye on the CO2 saturation as if it goes too high it can easily do harm to your fish and even kill them.

A simple way to get a visual indication of the CO2 saturation is with a CO2 Drop Checker. This is typically a small bulb that "samples" the water CO2 Saturation and contains some PH Indicator agent mixed with 4DKH Solution. Importantly the visual indicator is slow to react meaning the reading is from some hours prior to when you are looking at the colour of the liquid in the bulb.

Simply water saturated with CO2 will lower PH.  If the liquid in the bulb is Green then the CO2 levels in the aquarium are ok if yellow too high and if blue too low.

Making a 4dkh Solution

Ingredients List

  • Minimum 2.5litres of distilled or de-mineralised water

  • Bi-Carb soda or Baking Soda 2.4gm

  • Measuring Jug with minimum 500ml capacity

  • Accurate scales or measuring spoon (½ tspn = approx. 2.5gm)

  • Minimum 2ltr storage bottle

  • Minimum 600ml storage bottle


  1. Measure out 2ltrs of distilled water and put into a clean container or bottle.

  2. Put 2.4gm of Bi-Carb Soda (Baking Soda) into the 2ltrs of distilled water (you can use ½ teaspoon measure which is approximately 2.5gm of Bi-Carb Soda just scrape off with a slightly rounded object to reduce the measured amount a tiny bit)

  3. Cap the container/bottle and shake vigorously for about 2 minutes to make sure the Bi-Carb Soda has dissolved into the de-mineralised water completely. Mark this solution as 40 dkh

  4. Put 50ml of this new 40 dkh solution into a measuring jug

  5. Add 450ml of de-mineralised water and pour into minimum 600ml storage bottle.

  6. Test a sample from the 600ml storage bottle it should be very close to 4dkh


You can adjust your solution up by adding tiny amounts of 40dkh solution or down by adding small amounts of de-mineralised water. 

Test after each “add” as the dkh measure can move quickly.

This method is not 100% accurate although it is good enough method to make a solution for using in CO2 Indicator bulbs.