Ista CO2 Diffuser 2 in 1 With Bubble Counter Ceramic Plant Planted Aquarium Tank

Loosen the cap, take out the ceramic and then fill up the reservoir with clean water, then put back the ceramic and fasten the cap firmly. Connect the CO2 pipe to the CO2 inlet. The rate of CO2 production can then be adjusted by checking the number of bubbles using the built in bubble counter.

eBay