- Ideal for aquarium airline or CO2 tubing - 20 mm size is suitable for rimless aquariums - Great for use with CO2 injection systems or bubblers The 20 mm glass U Tube allows you to easily run airline (preferably silicone) for CO2 or for general air supply over the rim of the aquarium without bending or creasing. Made of strong clear glass, which minimizes visibility of tubing. Compatible with 6 mm (1/4") - Standard Airline Tubing.