How to Install Your Aquarium CO2 Drop Checker - GLA

1. Preparing the CO₂ Indicator Solution

The correct indicator solution for a CO₂ drop checker is a combination of 4 dKH reference solution and a pH indicator such as Bromothymol Blue.

With the drop checker upside down, fill the reservoir approximately halfway (or according to the manufacturer’s recommendation) using 4 dKH solution. A syringe or pipette is recommended for easier filling.

Next, add approximately 2–4 drops of Bromothymol Blue pH indicator until a strong transparent blue color is achieved.

How It Works

pH is a measurement of acidity or alkalinity in a solution. A pH indicator changes color depending on the pH of the solution.

KH (carbonate hardness) measures the concentration of carbonate and bicarbonate in water. A 4 dKH reference solution provides a known carbonate hardness standard without additional buffering compounds.

By combining a known KH value with a pH-sensitive indicator, the drop checker can provide a visual approximation of dissolved CO₂ concentration inside the aquarium. When the indicator solution reaches a green color, the aquarium is typically near an approximate CO₂ concentration of 30ppm.

• Blue: Lower CO₂ level
• Green: Approximate optimal CO₂ level (~30ppm)
• Yellow: Elevated CO₂ level

2. Installing the Drop Checker & Observing Color Changes

Place the drop checker inside the aquarium in a visible location with the open side facing downward.

After approximately one hour, the indicator solution should begin responding to aquarium CO₂ levels. For more accurate readings, allow approximately two hours for the solution color to stabilize before evaluating CO₂ concentration.

Observing the drop checker against a light or white background may improve color visibility and readability.

• Blue: Increase CO₂ injection
• Green: CO₂ level is within the target range
• Yellow: Decrease CO₂ injection

How the Drop Checker Measures CO₂

A CO₂ drop checker contains both an indicator solution and a small airspace that separates the solution from direct contact with aquarium water.

As dissolved CO₂ levels increase inside the aquarium, CO₂ diffuses into the airspace of the drop checker and is gradually absorbed into the indicator solution. This changes the pH of the 4 dKH solution, causing the Bromothymol Blue indicator to shift color.

As equilibrium develops between the aquarium water, airspace, and indicator solution, the color of the drop checker provides a visual approximation of aquarium CO₂ concentration.

When aquarium CO₂ levels decrease, the process reverses. CO₂ gradually diffuses back out of the indicator solution, causing the pH to rise and the indicator color to shift back toward blue.

When used properly, a CO₂ drop checker is one of the simplest and most effective tools available for monitoring planted aquarium CO₂ levels.