High quallity, easy to use bulkhead fittings that automatically stop the water at both ends when a line is disconnected. Just a quick twist connects or disconnects supply tubing. These are a little expensive but pay for themselves quickly if you need to change lines frequently. Great for flushing by letting quickly change from a reservoir with nutrients to one with just water. Also very helpful if you use different nutrients at different times. We also have adapters so you can connect these to 3mm or 1/4" line.