John Guest quick connect fittings make an easy and extremely secure connection. They can be used on copper tubing, but work best on plastic.
Be sure the end of the tubing is cut square, then push in until you feel the tube hit bottom.
Should you need to remove the tube, push in on the collet (the small plastic collar that encircles the tube) with your fingernail and pull the tube out simultaneously.
If the unit has been in use, you will not be able to remove the tube from the fitting until water pressure has been released.
When you remove a tube and reinsert it, it's a good idea to trim off a small portion of the end of the tube to assure a leak-free reconnection.
(whole house & well units)