For med - large sized aquariums (20 - 60g). Expandable with Expansion Module (sold separately) to suit larger aquariums. Each module can increase its capacity for another 20 - 30 gal. approximately. An unique patented dual-spray design: Upper turbo currents -> lower jet stream -> small bio-media to ensure 100% CO2 dissolution. CO2 enters at top and CO2-saturated water exits at bottom. Use with a small powerhead or internal filter. Bio-media included. Recommended flow rate is between 100 - 300 GPH. 2" diam. x 10" Ht. To expand these for bigger tanks add an expansion module for 8.99