When the drop checker is submerged in the aquarium, the carbon dioxide will outgas from the aquarium water into the airspace of the drop checker and will eventually be absorbed into the indicator solution. The carbon dioxide gas will seek a point of equilibrium between the indicator solution and the aquarium water. As equilibrium is achieved, the color of the indicator solution inside of the drop checker will change based on the concentration of CO2 in the aquarium water. Features: Maintaining proper Carbon Dioxide (CO2) levels in the planted. Aquarium is one of the important factors in the health of the planted aquarium ecosystem. A CO2 drop checker is an essential tool for monitoring CO2 levels in the aquarium. The CO2 drop checker is a reservoir designed to contain an indicator solution and an airspace. Long term monitor CO2 the color show: blue - insufficient CO2, green - sufficient CO2, yellow - excess CO2. Specifications: Test Solution Capacity: 10ML Drop Checker Material: Glasses Package Weight: 39g / 1.38ounces Package Size: 60 * 55 * 10mm / 2.36 * 2.17 * 0.39in Instruction: 1. Turn drop checker up-side-down. 2. Place 5 drops of test solution into the drop checker. 3. Refill aquarium water to the level line. The relationship between CO2 and pH: When CO2 dissolves in water a small portion of it combines with the water to form Carbonic Acid. As more CO2 dissolves and combines with water, the more the pH will drop. pH is an upside-down measurement, a pH rise from 6 to 7 indicates a tenfold decrease in the acid concentration, a pH drop from 6 to 5 indicates a tenfold increase in the acid concentration. Package List: 1 * CO2 Drop Checker 1 * Test Solution 1 * Sucker 1 * Suction Cup for Drop Checker