Features: 1. Made from high quality acrylic material, which is clear to watch the liquid quantity, studry and durable. 2. Concealed groove design: more convenient to open and fill into liquid. 3. Easy to use: this monitor liquid can be directly drop into and use, precise and reliable. 4. Help you add correct liquid quantity: with different color of liquid represents different concentration of CO2. BLUE means that CO2's concentration is too low. GREEN means it is normal. YELLOW means that CO2's concentration is too high so that it is harm to fish. 5. An essential tool for monitoring CO2 levels in the aquarium. Suitable for all kinds of size aquarium. Specification: Condition:100% Brand New Material: Acrylic Color: As Pictures Shown Co2 Monitor Size: approx.3.5*3.7cm/1.4*1.5in(W*H) Package list: 1 * Aquarium Co2 Monitor 1 * Aquarium Co2 Monitor Fluid(15ml) 1 * Suction Cup Note: 1. After adding the monitoring liquid, put it into the cylinder vertically, the color of the monitoring liquid will change along with the amount of carbon dioxide that has been put into. You can judge from the color to see if the co2 input is appropriate and adjust it. 2. Do not use the oxygen pump while using the monitor, or else it will affect the normal color display. Generally speaking, the grass tank is not need to add oxygen, for aquatic plants breathe out oxygen after photosynthesis, adjust the ratio of aquatic plants to fish and shrimp for let them live better together.