Features: brand new and high quality High quality of materials. Use with CO2 indicator. Easy to use and convenient to detect concentration of CO2. Suitable for all kinds of fish for tank. 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. Specifications: Material: 4dKH/PH Solution Color: As Pictures Show Style: 1pcs/2pcs Size (Approx.): 5x2cm/1.97x0.79in Weight (Approx.): 1pc-25g 2pcs-50g Capacity: 15ml/0.5oz Package Includes: 1 x CO2 Indicator Solution (1pc/2pcs) note: 1.Please allow 0-1cm error due to manual measurement. pls make sure you do not mind before you bid. 2.Due to the difference between different monitors, the picture may not reflect the actual color of the item. Thank you!