CO2 Drop Checker Guide

How a CO2 drop checker uses a pH indicator and a reference solution to show dissolved CO2 by color, why it lags, and how to use one.

What it is

A CO2 drop checker is a small glass indicator placed inside the aquarium that changes color according to the concentration of dissolved CO2. It provides a continuous visual reference so an aquarist can judge whether CO2 is low, around the target, or excessive.

How it works

The checker sits in the tank filled with a solution of water and a pH reagent, commonly bromothymol blue. CO2 from the aquarium water diffuses across an air gap into the dye solution and comes into equilibrium with it, changing the solution's pH and so its color. Because the reagent is separated from the tank water, it responds to the CO2 that has reached it rather than to the tank's general pH.

Reading the color

Bromothymol blue is yellow below about pH 6.0, green in the pH 6.0 to 7.6 range, and blue above about pH 7.6. As dissolved CO2 lowers the pH of the dye, the color shifts from blue (low CO2) through green toward yellow (high CO2). The best practice is to fill the checker with a reference solution of known carbonate hardness; a solution of 4 dKH is used as a standard so the color corresponds to a known CO2 concentration.

Why it lags

The color change is slow because CO2 must diffuse out of the water and reach equilibrium with the dye. It can take on the order of an hour or more for the color to stabilize, and the checker reacts especially slowly to drops in CO2. As a result it shows a delayed, time-averaged picture rather than the instantaneous level, and it is not a precise method, partly because color interpretation is subjective.

Use and setup

The checker is filled with the reference solution, leaving an air gap, and positioned where it can be seen and where it samples representative water. Because of the lag, the color is read with the delay in mind and confirmed over the photoperiod rather than reacting to it minute by minute. It is most useful combined with observation of plant and livestock response.

Maintenance

The reference solution and indicator are refreshed periodically, since the dye and reference can drift over time and give a misleading color. The glass is cleaned of algae or film that would distort the color, and the air gap is preserved so tank water does not mix directly into the reagent.