The specification of the trachea that can be connected with the joint is 4MM in inner diameter and 6MM in outer diameter. Reactor bottle cap to use on any soda/water bottle for a brine shrimp hatchery. Easy to install, and suitable for all kinds of DIY reactors. Use one side for the air pump, and the other side to release the brine shrimp into a container. Can add or control the amount of CO2 which is available to your fish and plants, add essential CO2 into your aquarium water.