CO2 Permanent Drop Checker

The CO2 Permanent Drop Checker is a glass or acrylic bell-shaped device that is hung inside the planted aquarium and filled with a 4 dKH reference indicator solution. A pocket of aquarium water trapped under the bell equilibrates with tank water CO2 through diffusion, causing the indicator solution inside to change colour based on pH — which directly reflects dissolved CO2 concentration.

The colour is interpreted using a simple three-zone guide: blue indicates insufficient CO2 (below 15 ppm), green indicates optimal CO2 (approximately 20–30 ppm), and yellow indicates excess CO2 (above 35 ppm) that risks fish stress. The response lag is 1–2 hours, making the drop checker a trend indicator rather than an instant-read device.

The drop checker must be refilled with fresh 4 dKH reference solution every 4–6 weeks as the solution gradually becomes exhausted or contaminated. The glass body is available in various pendant and suction-cup mounting styles.