Most likely the newest addition to measuring CO2 levels, a drop checker consists of an airspace between the liquid inside the drop checker and the water in the aquarium. Carbon dioxide readily diffuses outwards from water into the air; as such, the carbon dioxide in the aquarium will readily diffuse into the airspace in the drop checker. The liquid inside the drop checker contains a solution of known kH (i.e. 4 or 5 dkH) with an indicator (bromothymol blue (BTB)) which serves as a good indicator of CO2 dissolution. The CO2 that is in the airspace of the drop checker will readily diffuse into the drop checker solution, changing the colour of the BTB indicator.
Different drop checkers exist today. Red Sea makes one, ADA makes one, Cal Aqua makes several, some can be found on eBay etc. Of course, you can also DIY one yourself. They all work essentially the same, and perform the same function. Some people prefer one brand over the other due to aesthetics and/or ease of comparing the colours.
Here are some instructions for making your own DIY drop checker.
In addition to a drop checker, a reference solution is required when using a drop checker. This is usually a 4 dkH reference solution, but it can also be a 3 or a 5 dkH reference solution. The different dkH reference solutions will turn green (in the presence of BTB) at different CO2 levels (for example, the 4 dkH reference solution turns green at 30 ppm of CO2).
Sometimes, you may find instructions that come with purchased drop checkers to use aquarium water, distilled water, or even tap water. These instructions are incorrect and should be ignored.
If you do not use a reference solution, you will get incorrect results when using a drop checker.
Many people (mistakenly) believe that the pH/kH/CO2 relationship is the end-all for measuring CO2 levels. They believe that by measuring the pH and kH of the aquarium water, they will know their CO2 levels. However, this is not the case.
The pH/kH/CO2 relationship can only
be used if carbonates are the only buffers in the water. However, in the aquarium, there are other factors that will affect this kH reading (such as phosphate buffers). As a result, the pH/kH/CO2 relationship cannot be used with tank water, tap water, or distilled water.
When making a dkH reference solution, we are only adding carbonates to the water, so only here can we make valid conclusions using the pH/kH/CO2 relationship.
Instructions for making your own 4 dkH reference solution are also in the link above