Get your John Guest 3/8" QC X 1/4" QC Union Reducing Connector. Made of food grade plastic, they're high quality parts.
John Guest fittings are a quick and inexpensive way to make a connection for your reverse osmosis filters. They are strong and durable, can be used in commercial applications like soda fountains and commercial icemakers. They are Federal Safe Drinking Water Act compliant, as well as being very easy to install and put in use, making repairs fast and easy.