Commercial photo of drop checker
Drop checker
Green drop checker behind amano shrimp
Real-world green drop checker

A drop checker is a simple and reliable device for measuring the level of dissolved CO2 in aquarium water.  The drop checker hangs from the wall of the tank inside the tank and completely underwater, e.g. with a suction cup, and contains a liquid with a pH indicator dye in the “drop” portion which is connected to an open interface to the aquarium water at the bottom while maintaining an air gap in a U-bend between the aquarium water and the pH indicator solution.  There is an equilibrium between CO2 in solution in the water (in the form of carbonic acid) and the gas phase CO2 in the air gap.  The CO2 in the air gap can dissolve back into the aquarium water and equivalently into the drop checker solution such that eventually a steady state is reached where the concentration of carbonic acid in the aquarium and the concentration of carbonic acid in the pH indicator will be the same when then the amount of carbonic acid in the aquarium can be inferred directly from the colour of the pH indicator solution.  Usually bromothymol blue is the pH indicator and the drop checker solution is bromothymol blue diluted into a defined concentration of sodium bicarbonate at typically a “hardness” of 4 dKH.  It is convenient to purchase pre-made bromothymol blue in a 4 dKH solution that can be added directly into the drop checker.  We’re looking for as much dissolved CO2 as possible to support plant growth without have so much CO2 that it adversely affects animal life, generally accepted to be around 30 parts per million (PPM) dissolved CO2.  Under a typical drop checker setup with 4 dKH solution, a blue colour indicates not enough CO2, green indicates the right amount of CO2, and yellow means too much CO2.

Some notes:

  1. It generally takes a couple of hours at least for the drop checker to reach equilibrium CO2 levels with the aquarium water, so this is not a “rapid response” test – you leave the drop checker in the tank all the time. 
  2. At low CO2 the blue colour of the drop checker solution is also opaque.  As the drop checker becomes progressively more green towards yellow as CO2 levels rise, the solution will also become transparent so you’ll be able to see details in things behind the drop checker.  The real-world drop checker picture above exhibits this type of transparency.
  3. Even though at equilibrium the concentration of carbonic acid in the drop checker is the same as the concentration of carbonic acid in the aquarium water, the pH of the drop checker solution is not the same as the pH of the aquarium water.  This is because the pH of the aquarium water is influenced by many factors other than carbonic acid none of which affect the drop checker, and because the pH is also strongly influenced by the “hardness” of the water which is always 4 dKH in the drop checker but could be almost anything in the aquarium.
Coloured table of CO2 vs. drop checker solution dKH vs. pH in the drop checker
Drop checker colour chart

Here’s a chart I made that shows what colour the bromothymol blue solution in the drop checker will be, shown as a function of hardness of the water in the drop checker (usually set to 4 dKH – shown as the row headings) and the concentration of dissolved CO2 in the aquarium water (shown in the column headings).  People generally aim for 30 parts per million (PPM) CO2 dissolved in the water, so if you read the chart across at 4 dKH you can see how the central lighter and darker green colours (outlined in bold boxes) indicate a CO2 concentration of beween 20 – 40 PPM, which is generally fine.  The numbers inside the coloured boxes are the pH of the drop checker solution at equilibrium, which again is not the same as the pH of the aquarium water.

The older way to measure dissolved CO2 was to measure the pH of the aquarium water directly and to measure the dKH of the aquarium water directly, where you could then read off the dissolved CO2 levels from a pre-calculated chart.  Since it is quite difficult to get an accurate reading of either aquarium water pH or dKH, the drop checker solves these difficulties by having a defined dKH in the drop checker solution and using a very sensitive pH indicator dye.


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