A CO2 hanging drop checker is an elegant, non-submerged glass device used to monitor dissolved carbon dioxide levels in planted aquariums. By hanging it over the tank’s edge, it prevents algae from growing on the viewer and eliminates the need for suction cups.

How it Works & Colours:

Inside the bulb, you place a specialized indicator solution (typically bromothymol blue mixed with 4 dKH reference water). This liquid changes colour based on the gas exchange between the tank water and the trapped air pocket:

• Blue: Too little CO2
• Green: Ideal CO2 level for plant growth and fish safety
• Yellow: Too much CO2, which can suffocate fish

Setup & Maintenance:

1. Fill the bulb: Turn the hanging checker upside down and add about 15 to 20 drops of CO2 indicator solution into the reservoir.
2. Hang it: Carefully invert the checker and hang it over the rim of your aquarium so the main bulb remains outside the water and the open tube dips just below the surface.
3. Wait for readings: It typically takes about 1.5 to 2 hours for the solution to chemically react and change colour.
4. Maintenance: Replace the indicator fluid every 4 to 6 weeks to ensure the colour readings remain accurate.