This bottle cap is suitable for most kinds of DIY CO2 reactor. It is designed to fit most common PET plastic bottles.
It is easy to use: just screw the bottle cap on the plastic bottle. Since it is made entirely out of ABS plastic, the bottle cap does not rust and is safe for aquatic use.
Each bottle cap has two holes making it easier to add both airline tubing as well as other CO2 accessories (e.g. bubble counter).